HEMIASTER GAVERNOSUS. 5ST 



small re-entering angle in the posterior lateral interambulacra, follows gen- 

 erally the outline of the test ; it varies greatly in breadth in some specimens ; 

 the width of the fasciole is nearly as great as the width of the petals, in 

 others it does not equal the width of the poriferous zones. The genital 

 pores are small. There is no marked difference in the tuberculation of this 

 species from that of H. cavernosus, except that the tubercles adjoining the 

 petals are somewhat larger, compared to those of the intermediate spaces. 

 The shape of the actinostome and its position is essentially the same as in 

 H. cavernosus. The spines resemble those of H. cavernosus on the actinal 

 side ; the large tubercles carry long, straight, slightly curved, and club-shaped 

 spines, with a short groove at the extremity ; the other spines are similar 

 in structure, but shorter. 



Patagonia. 



Hemiaster cavernosus 



! Tripylus cavernosus Phil., 1845, Wieg. Arehiv., p. 347. 

 ! Hemiaster cavernosus A. Ag., 1872, Rev. Ech., Pt. I. p. 132. 



PL XXI'. f. i, 2. 



This species would at first glance, on account of its deeply sunken lateral 

 ambulacra, be taken as the young of Tripylus excavatus ; specimens of the 

 same size of the latter species show the presence of the lateral fasciole as 

 fully and clearly marked as in older specimens ; while the absence of this 

 fasciole, and the different shape qf the actinostome and of the posterior part 

 of the test, are sufficient to distinguish these species, belonging, as they do, to 

 different subgenera. The test is depressed ; outline from above pentagonal, 

 with rounded angles ; posterior extremity vertically truncated. Apical system 

 and vertex coincident; the apical system scarcely sunken below the level of 

 the surrounding interambulacral ridges. Anterior and posterior lateral am- 

 bulacral petals sunken, nearly of the same length ; anterior somewhat the 

 longest. Anterior groove very shallow, — a mere indentation of the anterior 

 part of the test. Peripetalous fasciole following in its general outline the 

 outline of the test, with a concave side in the lateral posterior interambu- 

 lacra. Test in profile arching regularly towards the ambitus ; actinal surface 

 slightly convex, with an indistinct rounded keel near the posterior extremity 

 of the actinal plastron. Actinostome small, broad, crescent-shaped near the 

 the anterior edge ; posterior lip but little prominent ; anal system compara- 

 tively large, longitudinally elliptical, pointed at the two poles. On the 



