Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/112

Rh Rostral plate.—A small triangular rostral plate with narrow, sharp, longitudinal median ridge has been seen in four specimens (see fig. 3, pl. 19; fig. 2, pl. 20); it is located in the median line between the antennae.

Dimensions.—The average length of an entire adult specimen is from 40 to 50 mm., the carapace being about 16 mm. in length with a width when flattened of 15 mm. The general proportions of the various parts are shown by figure 4, plate 27, of the 1912 paper.

Eyes.—The eyes are relatively large and placed at the end of a stalk or peduncle that projects from beneath and on each side of the rostral plate as seen from above. The peduncles are slender at their proximal end and expanded in a broad oval outline on the distal third of their length, the expanded section carrying the elongate oval visual surface; the peduncle appears to have had at least one joint at about the inner third of its length and to have been attached to a prostomium at its proximal end.

Cephalic appendages.—These consist of a pair of long jointed antennae that project forward beside and beneath the median rostral plate, and a pair of short lobelike antennules appear to be represented close to the eye and above the antennae (see fig. 3, pl. 19; fig. 2, pl. 20) in several specimens. Traces of three pairs of cephalic limbs have been observed but their structure and form are unknown.

Thoracic limbs.—A number of specimens have five strong thoracic limbs that extend from their union with the body trunk forward and outward beyond the edge of the carapace (see fig. 2, pl. 18); the distal joint is short and has three strong and two small curved spines projecting from its outer end and fine spines along its margin; the three next joints are rather short and spiniferous, but the detailed character of the remaining joints is unknown. The limb observed is assumed to be the endopodite of a biramous limb, but the exopodite was not developed or it was so small and delicate as not to be preserved in the fossil state.

Abdominal limbs.—Each of the abdominal limbs is represented by long, multi-jointed exopodites bearing long, slender filaments (see fig. 3, pl. 20). The proximal joint was probably short and without fringing filaments, but none of the specimens proves this to have been the case; the exopodites are rather large at the proximal end, tapering gradually to a slender, flexible terminal section; the filaments of the terminal section are sometimes gathered in tufts or bundles as shown by figure 3, plate 20. The filaments are usually flattened and matted together, but a few specimens show them to have been slender, cylindrical tubes similar to the filaments on the exopodite of Marrella