GESTEEA AISTD GEOTJPS OP THE ECHINOIDEA. 227 



Troscliel, defined Tripylus as Semiaster witli the ambulacra 

 surrounded by a very flexuous faaciole, with, a lateral fascicle 

 separating from it and descending under the vent ; ovarial pores 

 three or four. He remarked that the genus differs from Desoria 

 {Zimthid) and Schizaster in the regular cordate form and central 

 vertex. He again admitted Tripylus Philippi, Gray, in the 

 group, but unfortunately this species is a true Schizaster, and 

 also T. excavatus. 



In 1873 A. Agassiz, Revision, p. 588, placed Tripylus as a sub- 

 genus of Hemiaster, noticed the peripetalous fascicle v^ith con- 

 tinuous lateral and anal fascicles ; and in the specific description 

 of T. excavatus, the only recognized species, he mentions the 

 three huge genital openings. The deep ambulacra were neces- 

 sarily remarked upon. 



The same author, however, removed the species placed in 

 Faorina by Grray and mAhatus by Troschel, namely " cavernosus''' 

 and " australis,^'' back again into Hemiaster, and observed that he 

 could not find a trace of the subanal fascicle figured by Philippi. 



In ' Les Etudes,' Loven, 1874, pp. 13, 16, pi. xi. fig. 99, a draw- 

 ing is given of the apical system of Ahatus Philippi, Loven, 

 1871 {jion Grray, sp.), Ofversigt af K. Yet.-Akad. Forh. no. 8, 

 p. 1070 ; and two genital pores are prominent features, their 

 basal plates being separated by a long madreporite. On pi. xxix. 

 are plans of the test oi Ahatus Philippi Sbiid of the buccal and anal 

 plates ; and although it is evident that Loven considers Ahatus to 

 be a genus, the only distinction between it and Hemiaster, given, 

 refers to the two genital pores and the backward projecting 

 madreporite. 



A. Agassiz had the advantage of seeing Loven's type, and as 

 during the voyage of the ' Hassler ' many other specimens were 

 obtained, the results of their examination were pubKshed in 1874 

 (A. Agassiz, Zool. Eesults of the ' Hassler ' Expedition, p. 20, 

 pi. iv. figs. 4-8). It was shown that the number of genital pores is 

 variable, " as we find two or three quite indifferently ; the poste- 

 rior pair of genital openings is always present ; if a third exists 

 it is the right anterior one usually, but sometimes the left." A. 

 Agassiz terms this form Hemiaster, and gives Ahatus as a syno- 

 nym. He places Tripylus cavernosus as a Hemiaster, and only 

 recognizes Tripylus excavatus, Phil. 



Mr. Edgar A. Smith (1879, Phil. Trans. Roy. Soc. clxviii. 

 p. 271) placed under Hemiaster the Ahatus cavernosus, and its 

 synonyms (generic) Brissus, Tripylus, Faorina. 



15* 



