226 PEOF. p. M. dtjncan's keyision oe the 



with four genital pores. Ambulacra petaloid, limited, concave, 

 divergent. Peristome strongly bilabiate. A peripetalous fasciole." 



This somewhat imperfect diagnosis would contain the Mesozoic 

 species and some recent forms ; but it excludes certain species of 

 Cretaceous, Tertiary, and Eecent ages, if too great a value is 

 placed upon the position of the madreporite, the number of 

 genital pores, and the existence or not of a lateral fasciole, which 

 may be more or less transient. 



It is necessary to revise the genus, and to consider the evidence 

 afforded by recent discoveries. It is now beyond doubt that 

 some species of Semiaster are viviparous, or rather that the 

 female receives the young which have not been freely swimming 

 Plutei into her ambulacral petals ; that during growth the peri- 

 proct is at first above a fasciole, which environs the petaloid 

 parts of the ambulacra, and that as the opening becomes lower 

 down and more posterior, it crosses and often divides the fasciole. 

 Thus badly limited lateral and infra-anal fascioles occur or not. 

 During growth the depth of the petals, the position of the 

 madreporite, and the number of genital pores varies. (See A. 

 Agassiz, Eeport on ' Challenger ' Echini, p. 179.) 



In 1845 Philippi, in Wiegm. Archiv, p. 344, pi. xi., diagnosed 

 Tripylus excavatus, T. cavernosus, and T. australis, each with 

 three genital pores only. He placed Tripylus as a subgenus of 

 Spatangus. In 1851 Troschel, Wiegm. Archiv, p. 72, wrote on 

 the " genus " Tripylus, and defined it as follows : — Test cordate, 

 suborbicular, convex ; ambulacra deep, the anterior pair sub- 

 transverse ; the dorsal pole subcentral. Genital jDores three. A 

 fasciole including the ambulacra. 



Troschel divided his genus into : — 



1. Samaxitns — a line of fasciole passing under the periproct. 

 The type was Tripylus {ELamaxitus) excavatus, Philippi. 



2. Atrapus — the infra-anal fasciole incomplete. Tripylus 

 grandis, Troseh. 



3. Ahatus — no infra-anal fasciole. Species Abatus cavernosus 

 and A. australis. 



In 1851 Gray (Ann. & Mag. Nat. Hist. ser. 2, vol. vii. 

 p. 132) diagnosed Faorina, and included F.cTiinensis, F. antarctica, 

 and F. cavernosa. The last two species belong to the subgenus 

 Tripylus of Philippi. He also noticed Tripylus to be a genus 

 which included a new species, Tripylus Philippi. 



Gray in 1855, Cat. Eec. Ecli. Brit. Mas. p. 58, without noticing 



