64 TH. MORTENSEN, (Schwed. Siidpolar-Exp. 



would even justify making it the type of a separate genus; but since it otherwise 

 agrees so closely with the other species I have thought it better to refer it to the 

 genus Plexechimis, which represents, in any case, its nearest relation. 



The marked deepening of the anterior ambulacrum and the actinostome is a 

 highly interesting feature, pointing distinctly towards the Pourtalesiae. This species, 

 much more than the tvvo other species hitherto known of the genus, tends to mark 

 the genus Plexechimis as a connecting link between the Urechinidce and the Pour- 

 talesiidcz; but still the urechinid affinities are the most prominent. (Cf. »Ingoh> 

 Echinoidea. II, p. 57—58.) 



Amphisternata. 



Fam. Spatangidae. 1 



The South American Spatangoids of the Abatus-group have a very interesting 

 and rather intricate history. The first author to treat of them was PHILIPPI, who in 

 1845 ' n n ' s paper »Beschreibungen einiger neuen Echinodermen, nebst kritischen Be- 

 merkungen tiber einige weniger bekannten Arten» (Arch. f. Naturgesch. XL I p. 344) 

 establishes three species, viz. Tripylus excavatus, cavernosas and australis. While 

 L. AGASSIZ & DESOR in their ^Catalogue raisonne des Echinides* (1846) refer Tr. 

 excavatus to Agassizia and the two other species to Brissopsis, TROSCHEL goes another 

 way, establishing a subgenus, Hamaxitus, for Tr. excavatus and another subgenus, 

 Abatus, for cavernosus and australis (»Uber die Gattung Tripylus*. Arch. f. Natur- 

 gesch. 185 1). Next Gray in his »Catalogue of the Recent Echinida» (1855) refers 

 the two latter species to his genus Faorina, to which genus he likewise refers the 

 species antarctica described by him in 185 1 (Descriptions of some new genera and 

 species of Spatangidse in the British Museum. Ann. Nat. Hist. 2 Ser. VII. p. 130). 

 Later on (1872) A. AGASSIZ in the »Revision of Echini» refers the same two species 

 to the genus Hemiastcr {Faorina antarctica being made a synonym of H. caverno- 

 sus), and in the »Hassler»-Echini (1874 p. 20) he points out that H. australis is in 

 all probability the young of H. cavernosus. Subsequently, in the »Challenger» Echi- 

 noidea (188 1 p. 184) AGASSIZ has come to the conviction that »there seems but 

 little doubt the species which have thus far been distinguished as Hemiaster austra- 

 lis, Hemiaster philippii 2 and Hemiaster cavernosus are all different stages of growth 



1 On referring the following forms to the family Spatangidce I wish to state expressly that it is not 

 meant as an expression of my views on the classification of the Amphisternata. I hope to be able to set 

 forth my views hereon in the Part II of my >Siam-EchinoiJea». 



7 Meantime established by LoVEN in 187 1. 



