TRIPLECHINIDAE. 487 



TRIPLECHINIDAE. 



Subfamily Triplechinidae A. Agass., 1872, Rev. Ech., Pt. II. 



PHYMOSOMA. 



Cyphosoma (Agass.). 1840, Cat. Syst. Ectyp. (non Mann.). 

 Phymosoma IIaime, 1853, D'Akcii. et Haime, An. foss. Indc. 



The teeth of this genus, representing a large number of fossil species of 

 this and allied genera, of which there are now living but few representatives, 

 do not present any striking features to distinguish them from the teeth 

 of the Echinidae proper ; they are not in the least allied to the teeth of 

 Cidaridae and of Diadematidae. 



The value of the presence or absence of crenulation of the primary tuber- 

 cles as a generic character has been frequently discussed, and, owing to the 

 difficulty of clearly distinguishing the presence of crenulation in fossil species, 

 its value and importance have not been understood. As far as we can judge 

 from the recent genera with crenulated tubercles, — Diadema, Phymosoma, 

 Salmacis, Temnopleurus, and many genera of Spatangoids, — it is a feature 

 which appears early and seems quite characteristic, but is frequently indis- 

 tinct or obliterated. We can only guess at the function of this crenulation ; 

 it seems connected with the rapid and easy movements of the large or 

 long spines attached to them, which require considerable muscular force 

 and good points of support for the attachment of the muscular sheath at the 

 base of the spines. 



Phymosoma crenulare 



! Glyptocidaris crenularis A. Ac, 18G3, Proc. Acad. N. S. Phila. 



! Phymosoma crenulare A. AG. f 1872, Rev. Ech., Pt. I. p. 151. r r>/ VVVT/T/T f 



PI. VI. f. 2-8 ; PI VIP. f. 6, 8, 9; PL XXV. f. ss; PI. XXXVI f. io, n; 



Test depressed, slightly pentagonal, flattened on the actinal side. Actinal 

 opening moderate ; actinal cuts slight. Actinostome covered by a thin 

 membrane, thickly covered by elliptical plates, and their intervals filled by 

 smaller elongated plates occupying the whole space. The median interam- 

 bulacral and ambulacral spaces are bare from the abactinal system nearly to 

 the ambitus. The two principal vertical rows of primary tubercles diminish 



