Page:EB1911 - Volume 27.djvu/298

Rh implies the antecedent existence of much simpler types, and refers the origin of life to a date immeasurably distant from that at which we have actual proof of the existence of animal and vegetable organisms.

However different in structure Trilobites may be, they all agree in possessing a head-shield usually semi-circular in shape, which results from the fusion of apparently five segments, and bears, except in some blind forms, a pair of large reniform compound eyes like those of the king-crab (Xiphosura). This head-shield is succeeded by a varying number of free segments, each of which consists of a medium convex tergal piece and a pair of arched lateral plates, the pleura, of which there is one on each side. The terga and pleura of each individual segment overlap those of the segment that serially succeeds it. The mid-region of the body, composed of jointed segments, is followed by a larger or smaller region consisting of fused segments and termed the pygidium or caudal shield, which in some cases is as large as the head-shield itself, in other cases much smaller. When the pygidium is large and composed of many segments, the number of free body segments is correspondingly reduced, and vice versa. It is with respect to this number of segments that respectively constitute the pygidium and the mid-region of the body that Trilobites differ most markedly from each other; and it is a singular fact that the extremes in structural organization in this particular to be met with in the Trilobita are found side by side in strata of Cambrian age. In Paradoxides, for example, there are about twenty freely movable segments followed by a very short and small pygidium, whereas in Agnostus the freely movable segments are reduced to two and the pygidium is as large as the cephalic shield. In this genus the number of segments composing the pygidium is obscured, as also it is in the genus Illaenus, which has as many as ten movable segments preceding the large semi-circular pygidium; but in such forms as Ogygia and Asaphus, which have about eight free segments, the sutural lines on the pygidium indicate that it is composed of about a dozen or more segments. Somewhat resembling Agnostus is Microdiscus, with four movable segments and a large pygidium consisting of about five fused segments, the lines of union between the latter being clearly indicated.

The tergal and pleural elements of the pygidium are generally well marked. They are also well marked on the cephalic shield, the tergal elements being represented by a median axial elevated area showing indistinct signs of segmentation, and a lateral unsegmented plate, the gena, which carries the eyes. The posterolateral angles of the gena are commonly produced into spiniform processes, which may project backwards beyond the middle of the body as in Paradoxides, or considerably beyond its posterior termination as in Trinucleus or Ampyx. The latter is further remarkable for having the median area of the head-shield, the flabellum, produced into an anteriorly directed spike.

For many years only the dorsal surface of Trilobites was known, nothing having been ascertained of the ventral surface and appendages. Comparatively recently, however, specimens have been obtained with the ventral surface exposed, revealing the number and structure of the limbs. A pair of the latter was articulated to the sides of a moderately wide dorsal plate on each segment of the body, and similar limbs were attached to the ventral surface of the head-shield behind the mouth. Each of these limbs was two-branched, the external branch consisting of a slender fringed flagellum possibly respiratory in function, and the inner of a normal jointed ambulatory leg. These two branches arose from a common basal segment or coxa, the inner surface of which was produced into a strong process underlying the external area. In the region of the mouth the basal segments were armed with teeth and subserved the purpose of mastication. As in all Arachnida there is only a single pair of appendages in front of the mouth, and these were one-branched, long and filiform and acted as antennae. Under the pygidium or caudal shield the appendages were much shortened, and their main branch consisted of broader and flatter segments than those of the preceding limbs.

Such was the structure of the appendages in Trilobites belonging to the genus Triarthrus; but considering the great structural differences that obtain between Triarthrus and many other genera, it would be rash to assume that there were not corresponding differences in the structure of the limbs. It must not indeed be assumed that those of the first pair were in all cases antenniform.

It is probable that no satisfactory classification of the Trilobites will be proposed until the limbs of most of the genera have been examined. Up to the present time all attempts to arrange the genera in natural and definable groups have failed to meet with general approval; and this criticism must be extended to Beecher’s subdivision of the class into three orders, named Hypoparia, Proparia and Opisthoparia, based upon the form and position of a groove, the so-called genal suture, which marks the lateral portion of the head-shield. In the majority of Trilobites this groove passes backwards from the anterior or anterolateral edge of this plate to its posterior or postero-lateral border, dividing it into an inner portion continuous with the flabellum and fused tergal regions, and an outer portion bearing the eye. Those genera, like Paradoxides, Olenus, Asaphus, Phillipsia and others, in which this groove cuts the posterior edge of the head-shield on the inner side of its angle are referred to the Opisthoparia; those, like Dalmanites and Phacops, in which it cuts the lateral border in front of the posterior angle, belong to the Proparia. But in certain genera, like Conocoryphe, Calymmene and Triarthrus, it cuts the margin of the head-shield so close to the posterior angle that the distinction between the two groups practically breaks down. To the Hypoparia belongs a comparatively small number of genera, like Trinucleus and Aquastus, in which this groove or genal suture is beneath the margin of the head-shield and does not appear upon its upper surface.

In external form Trilobites are not unlike Isopod Crustaceans, especially the terrestrial species commonly called “woodlice”; and until the nature of their appendages was known, it was thought by some authorities that the two groups might be related. Like the woodlice they were capable of rolling themselves up into a ball, many specimens having been found fossilized in this state, with the pygidium pressed tightly against the head-shield. There is very little doubt that they lived at the bottom of the sea, feeding upon worms or other soft marine organisms, crawling slowly about the sandy or muddy bottom and burying themselves beneath its surface when danger threatened. That these animals were widely distributed in former times is proved by their occurrence at the present day in palaeozoic fossiliferous strata both of the northern hemisphere and of Australia; and despite the fact that their remains have not been found in rocks of the Mesozoic or Kainozoic epochs, it was conceived to be possible that living specimens might be dredged from the sea-floor during the exploration of the ocean depths undertaken by the “Challenger” expedition. Needless to say this faint hope was not borne out by results. (R. I. P.)&emsp;  TRIM, a market town and the county town of Co. Meath, Ireland, on the upper waters of the Boyne, 30 m. N.W. by W. from Dublin on a branch of the Midland Great Western railway. Pop. (1901), 1513. The county buildings are here; monthly fairs are held, and there is considerable trade in corn and flour; but the chief interest of the town lies in its historical associations and remains, enhanced by a beautiful situation. It was the seat of a very early bishopric. A Norman tower, called the Yellow Steeple, is supposed to mark the site of St Patrick’s Abbey of St Mary. Two gates remain from the old town walls. King John’s Castle (incorrectly so called, as this monarch only resided here on the occasion of a visit) was originally founded by Hugh de Lacy in 1173, but a later date is assignable to the greater part of the magnificent moated building, of which the keep, flanking turrets, drawbridge, portcullis and barbican, still testify to its former strength, which was augmented by its frontage to the river. Other smaller fortified buildings are Talbot’s and Scurlogstown Castles; the former erected by Sir John Talbot, lord lieutenant of Ireland in 1415—afterwards earl of Shrewsbury, the latter dating from 1180. About a mile east of the town, the ruins of the abbey of St Peter and St Paul occupy both banks of the river. These include the transitional-Norman cathedral on the north bank, and a castle, guarding the crossing of the river, on the south, together with a chapel and other remains. North of the town ruins may be seen of a Dominican friary of the 13th century. The tower of the old parish church dates from 1449. In the annals of Trim many famous names have a place; Humphrey of Gloucester and Henry of Lancaster were imprisoned here by Richard II. before Henry came to the throne; and Richard, duke of York, and father of Edward IV. held court at the castle, where also several Irish parliaments met until the middle of the 15th century, and a mint was established in 1469. The residence in a house in Dublingate Street of the famous duke of Wellington is commemorated by a Corinthian column and statue. Trim is governed by an urban district council. It was incorporated by Edward III., and returned two members to the Irish parliament until the Union in 1800.  TRIMMER, JOSHUA (1795–1857), English geologist, was born at North Cray in Kent, on the 11th of July 1795. He was son of Joshua Kirby Trimmer of Brentford, and grandson