98 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



Family Rinkceratid^. 

 Genus Thrincocekas, Hyatt, 1893. 



Thrincocekas Hyatti, sp. nov. Plate XXVI, figs. 1 a — g. 



Description. — Shell discoid, compressed, composed of nearly three whorls, the 

 sides of which are entirely exposed in a shallow umbilicus having a small, central 

 vacuity. 



The whorls increase rather slowly in diameter, the last one in the adult shell 

 becoming detached from the preceding one for a short distance. The body- 

 chamber occupies about half a volution. The section is subquadrangular and 

 hexagonal; the peripheral area is depressed but somewhat convex; the sides are 

 flattened and converge from the umbilical angle towards the periphery. The 

 umbilical slopes in the adult shell are moderately steep, the angle connecting 

 them with the sides not distinctly defined. There is no distinction in the young 

 shell between the sides and the iimbilical slopes, the whorls being there evenly 

 rounded. The peripheral area in the adult, on the other hand, is separated from 

 the sides by a well-defined angle, but whether this is the case in the young shell I 

 have no means of judging, as that part of the shell is covered up by the later 

 volutions in the specimens available for study. 



The impressed zone, which is shallow, is marked out by obscure, rounded 

 elevations (PI. XXVI, fig. 1 c). The presence of this zone indicates, in the 

 young shell, a slight median elevation which is less distinct in the adult. 



The septa are moderately distant; where the diameter of the whorl is 15 mm. 

 they are 5 mm. apart upon the sides of the shell, and this distance scarcely varies 

 in the entire whorl, the diameter of which is 63 mm. The sutures form a 

 somewhat deep, backwardly directed lobe upon the sides, cross the umbilical 

 margin with an ac^te angle, and form a rather shallow sinus upon the periphery. 



The siphuncle, as seen in the young shell, is a little above the centre of the 

 septum (PL XXVI, fig. 1 g). 



The ornamentation of this species is very elaborate. The test is completely 

 covered with fine, acute, longitudinal ridges ; in the adult shell these are widest 

 apart upon the umbilical declivities, where there are five or six upon the last 

 whorl; on the sides they become closer, about twelve may here be counted; upon 

 the periphery they are still finer, there being sixteen or seventeen, of which four 

 or five are crowded together on each side of the median zone, where there are six 

 more widely spaced ones. The ridges are developed also upon the impressed 



