Page:Synopsis of the Exinct Batrachia and Reptilia of North America. Part 1..pdf/107

 AND AYES OF NORTH AMERICA. 101 the posterior cutting edge being directed outwards. The anterior alveolus is shallower than the second, and this shallower than the third, which gives an oblique slope to the fractured margin of the bone, and suggests the application of another skeletal piece. This I suppose to be the premaxillary, as the bone is externally too fiat to permit the median premaxillary suture to occupy that position. The upper portion clay be related to the margin of the nares. A series of five foramina extends along the outer face of the bone opposite the middle of the depth of the alveolæ. The alveolæ are directed more anteriorly from behind forwards.

Lines. Depth of alveolus, Length erown of successional tooth, Length piece embracing four alveohe, 61

Mandible.—One portion from the anterior part of the ramus. The latter measure three inches in depth from the outer alveolar border, which is a little more elevated than the internal, and 1.5 in. in thickness at the fractured edge. A longitudinal series of vascular foramina extends along the middle of the external face. The teeth are implanted in deep alveolre, and bad transversely oval eompiessed fangs; the sections of the crowns of teeth from different portions of the ramus differ. Two from the anterior region are considerably recurved, the concave or posterior edge detieulate to the base of the enamel, the anterior aspect minutely serrate, two fifths the length from the tip. Section at this point lenticular, lower down the anterior face becomes broader and rounded, giving a rounded cuneiform seetion. Throughout, one face is more convex than the other. A young posterior tooth yet in the alveolus (no. 3) is less recurred, subacute, and of more lenticular section, having both edges denticulate to the base of the enamel. Fangs hollow, the pulp cavity capacious but rapidly diminishing and short.; the east sulphide of iron and marl.

Inches. No. 1; total length (fang broken), length of enamel, width below, anterior diameter, 2 33 1.83 S33 .433 No. 2; length of crown (tip restored). 1.S73 anterior diameter, .5 No. 3; length of erown, width at base, 2.125 .688

Larger teeth are indicated by fragments. The development of the teeth has apparently proceeded as in Megale-saurus. The development of the dental papilla takes place within a niche of the alveolus, between it and the inner mandibular or maxillary wall. Small serate cusps are found in this position beneath but a thin stratum of bone. In one situation a second successional tooth occupies a position between the primary cusp and the functional tooth. and is about intermediate in size between them. These successional teeth then increasing in size, by a horizontal move-ment, transverse to the cranial axis, place themselves close to the fangs of the functional teeth, into whose places they gradually rise. An absorption of the dental wall probably prepares the older tooth for shidding, at which time the apex of the successional tooth is ready for use.*

Vertebne.—No cervical or dorsal vertebrae were preserved ; we have only as yet sacrals, and numerous caudals. All are much constricted medially, or hour-glass shaped, the centrum cylindrical in section throughout in most of the caudals, the anterior of the latter and the lumbars of deeper vertical than transverse diameter throughont. The articular surfaces are moderately shallow biconcave in all, most strongly in the subproximial caudals. The neural arches attached by permanent snture, and inferior surfaces for articulation of chevron bones. The caudals offer indication of neural spines ; their traces are on the majority low, and of considerable longitudinal extent. Artienlar

AMER!. PHILOS°. SOC.—VOL. XIV. 26
 * Dcslongchamps figures a tooth as doubtfully belonging to Pooellopleuruni. It resembles that of a Crocodilian, and probably belongs to a species of that subclass. Ile states that Megalosaures-like teeth occur in the strata in which Poeciloplcurum MIS found. There Is now much reason to believe that the lattor are the true teeth of the genus in question.