SCHIZASTEB LIMICOLA 77 



'Schizaster (.Periaster) limicola a Ag. 



Schizaster (Periaster limicola 4.GAS8. Bull. M. C. Z., V , No. 9, p 193, PL 111.. 1878. 



PL XXVI. Figs. 5,6. 



This specie- belongs to the generic group of Schizaster separated as Peri- 

 aster ly D'Orbign} from the genuine Schizaster. The test is quite globular, 

 when seen from above the outline is somewhat angular, the posterior ex- 

 tremity is vertically truncated, with a slight keel between the posterior 



petals near the apex; the anterior extremity has a shallow amhulacral 



groove, and i< vertically truncated from the edge of the peripetalous fasciole. 



The lateral anterior petals are nearly twice as long as the posterior one. ,-is 



well as broader. The petals are all about equally sunken. The peripetalous 

 fasciole has the shape ami position of that of Schizaster proper; the anal 

 fasciole is narrow, extending only a short distance on the sides of the test. 



The test is thickly covered with primary tubercles of a uniform size, car- 

 rying short, slightly Curved spines on the sides; the tubercles are somewhat 

 re crowded within the fasciole and on the ahactinal region of the odd pos- 

 terior interambulacral area. The coronal plates between the fasciole and the 



anal system are < paratively hare, having only few tubercles on the outer 



edges, Ou the actinal side the primary tubercles are larger, more distant, 

 except on the actimil plastron, where they are closely crowded, and carry 

 longer, larger, ami curved primary spines. The actinal lip is very sharp and 

 prominent ; the actinal membrane carries one large exterior row of plates 

 round the anterior edge of the actinostome, and smaller irregular plates over 

 the rest of the actinal surface. The anal system is comparatively small for 

 so large a species. Tin' separation of this group of Schi/aster a- a generic 

 type Beems very doubtful. It is based solely upon the more globular outline 

 of the test, and the discontinuity of the latero-anal fasciole. It max he con- 

 venient to form a subgeneric group with these characters; hut when we 

 attempt to draw the line between Buch forms as 8. ventricosvs and S 

 it becomes most difficult, if not impossible, t<> draw the line of demarkatiou 

 between the generic and Bubgeneric types. 



We lind the same difficulty in attempting to define the oumerous generic 

 subdivisions indicated byTroschel lor the genus Tripylus and its allied forms, 

 which pass insensiblj to Faorina ami the like, while in Schizaster the devia- 

 tions are in the direction of modifications passing into Periaster, Epiaster, 



