236 PYRINA^ 



Pyrina Desmoulinsii, (FArcUac, 1847, PI. LIV, fig. 2 a — /. 



Pykina Desmoulinsii, d'ArcMac. Mem. de la Soc. Geol. de France, 2e serie, tome 

 ii, p. 297, pi. xiii, fig. 4, 1847. 



— — Agassis et Besor. Catal. raisoime, p. 92, Module T. 86, 



1847. 



— — d'Orbigny. Prodrome, t. ii, p. 178, Etage No. 651, 1847. 



— — Woodward. Mem. of the Geol. Surv., Organic Remains, 



Decade v, pi. vi, fig. a, 1856. 

 Pyrina Prattii, Forbes. Morris Catal. Brit. Foss., p. 88, 1854. 



— Desmoulinsii, d'Orlignij. Paleontol. Fran^aise, tome vi, p. 4G7, pi. 981, 



figs. 7 — 11, 1855. 



Diagnosis. — Test tumid, oblong ; ambitus inflated ; posterior border slightly emargi- 

 nate ; upper surface depressed ; apical disc small, nearly central ; base concave in the 

 middle and pulvinated at the border ; mouth-opening oval, oblique, and nearly central ; 

 periprocte elliptical, supra-marginal, nearer the upper than the under surface. 



Dimensions. — Antero-posterior diameter one inch ; lieight half an inch. 



Description. — The test of this rare Urchin is oblong or elliptical, regidar and 

 symmetrical, depressed on the upper surface, inflated round the sides, and concave near 

 the centre of the under surface. The summit is sub-central, nearer the anterior than the 

 posterior border. The ambulacral areas are narrowly lanceolate ; the poriferous zones are 

 linear and depressed, and the pores minute, unigeminal, and placed in obhque pairs. lu 

 passing across the base the geminal pores become more and more oblique, until they fall 

 into a single file and terminate around the peristome. The plates of both areas support 

 a number of small equal-sized tubercles ; in fig. 2 e is shown their mode of arrange- 

 ment on the ambulacral and inter-ambulacral areas ; they have a quincuncial disposition on 

 the plates. The areal space around each is sharply defined, the tubercle is raised on a boss, 

 and its summit is perforated. The intermediate surface of the plates is covered with a 

 very fine microscopic granulation (fig. ^ (/). The tubercles at the base are much more 

 developed than those on the upper surface of the test. Fig. 2 h and fig. 2 i show the 

 basal tubercles magnified ; the areal space is deeply excavated out of the structure of the 

 plate, and the tubercle is larger than those on the upper surface at fig. 2 i. A portion of 

 the test near the mouth-opening is shown with the arrangement of the zones and the 

 disposition of the tubercles in this region. 



The mouth-opening is large, elliptical, elongated in the direction of its greatest axis, 

 slightly oblique (fig. 2 h), and situated immediately beneath the organic summit. 



