I 
22 PODOCIDAPJS SCULPTA. 
the actinal edge of the test. In a specimen of S. Paitersoni, measuring 14 
mm. in diameter, we find only two or three rows. The granulation of the 
actinal plates is coarser in /S'. rampina than in S. Pattersoni. 
The gills in specimens measuring 7-8 mm. in diameter are well developed 
as a short five-forked appendage covered by few pigment cells. In smaller 
specimens measuring 3 mm., the gills only fork once. 
The more we examine the Saleni^e, the more we are inclined to consider 
the group as holding a position intermediate between the Cidarida; and 
the Echinidas. For while the general structure of the coronal plates of the 
actinal and abactinal system, of the spines, and of the papilla?, recalls the 
Cidaridag, yet the structure of the teeth, the presence of gills with actinal 
cuts for their passage, and the existence of spha;ridia, are all features which 
associate them with the Echinidce proper. 
\ Arbacia punctulata Gray. 
Yucatan Bank. 14-84 fathoms. 
^ Podocidaris sculpta A. Ag. 
^^ Off Morro Light. 250-400 fathoma. 
It is not out of place while speaking of Podocidaris to call attention to 
the remarkable genus Tiarechinus of Neumayr,* one of the most character- 
istic embryonic genera I know. This diminutive Sea-urchin from the Trias 
of St. Cassian represents the young stages of Podocidaris at a time when 
neither the abactinal system nor the plates of the interambulacral area have 
become specialized. The whole abactinal part of the test appears from the 
figures of Neuma}^' to be still in the condition preceding the division of the 
abactinal system into an anal system and a genital ring, before the formation 
of the plates of the anal system or the division of the anal ring into its 
component parts. There are very faint indications of what I take to be the 
dividing lines between four genital plates in the figure of Neumayr. The 
actinal surface, on the contrary, is far more developed ; the large px'imary 
tubercles of the interambulacral areas, and the structui-e of the ambulacral 
system, agree most strikingly Avith the condition of the actinal surface of 
young stages of Podocidaris and Arbacia. Compare the figures I have given 
in the Revision of the Echini, Plates IV., V., and Figs. 68, 69, p. 734. 
* See the figure given in Plato II., Vul. LXXXIV. 1 Abth. p. 169, Sitzungsb. d. K. K. Akad. d. 
Wiss. Math, Nat. CI., Wien, 1882. 
