22 IIEMIASTER CAVERNOSUS. 



be different stages of growth ; II. australis corresponding to the younger 

 stages of II. cavernosus, with its deeply sunken lateral ambulacra. Unfortu- 

 nately sufficient material is not available to settle this question, as the 

 species of Hemiaster from South America arc extremely rare in the collec- 

 tions I have examined. Very small specimens of H. Philippii are quite 

 cylindrical (.75 mm ' in length) ; they go through changes of form very similar 

 to those described in the growth of Brissopsis up to the youngest stages 

 figured here, when the gradual sinking of the ambulacral petals commences. 



Lat 37° 42', South; Long. oG 3 20', West 44 fathoms. 

 Lat 51° 2G', South; Long. 68° ■> . West 55 fathoms. 

 Off CajKt Dos Bahias (East Coast Patagonia), oj fathoms. 



Hemiaster cavernosus 



I Tripylus cavernosus Pun... 1845, Wieg. An-hiv. 



: Hemiaster cavernosus A. Ac;., 1*72. Rev. Ech., Ft. I. p. 132. 



One large specimen collected at Port Gallant. 



Tripylus excavatus 



! Tripylus excavatus I'ltiL., 1845, VTieg. An li. 



With the southernmost specimens of Agassizia excentrica were collected a 

 couple of small Spatangoids, which 1 am unable to determine satisfactorily. 

 They resemble, in general outline and appearance, Agassizia. but without 

 having the characteristic structure of the anterior lateral ambulacra of that 

 genus ; the course of the peripetalous and lateral fascioles is the same, as 

 well as the general proportions of t lie ambulacral petals. 



The changes undergone during growth in Hemiaster Philippii, which have 



I n described above, seem to poinl to the probability of these small Spatan- 

 goids being the young of Tripylus e\ca\atus. They have all the structural 

 features of the genus, except the deeply sunken lateral ambulacra. The 

 largest specimen, measuring aboul 7""" long, diam., differed from the smaller 

 one by having slightly longer posterior lateral pctaloid nmbulacrn, already 

 slightly depressed; while in the younger specimen the lateral ambulacra 

 were flush with the test, as in Agassizia, and in the young of II. Philippii. 

 It seems probable, therefore, that in genera with depressed or deeply sunken 



petal- the young have, in their earlier Btages, petaloid ambulacra tlusli with 



the te-t. In Schizaster canaliferus the lateral posterior ambulacra are but 



slightly sunken, even in the largest specimen-, while the anterior ambulacra 



