22 PODOCIDARIS SCULPTA. 



the actinal edge of the test. In a specimen of S. Pcdiersoni, measuring 14 

 mm. in diameter, we find only two or three rows. The granulation of the 

 actinal plates is coarser in 8. varispina than in 8. 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 Saleniae, the more we are inclined to consider 

 the group as holding a position intermediate between the Cidaridae and 

 the Echinidae. For while the general structure of the coronal plates of the 

 actinal and abactinal system, of the spines, and of the papilla?, recalls the 

 Cidaridae, yet the structure of the teeth, the presence of gills with actinal 

 cuts for their passage, and the existence of sphaeridia, are all features which 

 associate them with the Echinidae proper. 



Arbacia punctulata Gray. 



Yucatan Dank. 14-S4 fathoms. 



Podocidaris sculpta A. Ag. 



Off Morro Light. 250-^00 fathoms. 

 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 tit 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 Neumayr 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 primary 

 tubercles of the interambulacral areas, and the structure of the ambulacral 

 system, agree most strikingly with the condition of the actinal surface of 

 young stages of Podocidaris and Arbacia. Compare the figures 1 have given 

 in the Revision of the Echini, 1'lates IV., V., and Figs. 68, 09, p. 734. 



* See the 6gure given in Plate II.. Vol. I.. XXXIV. 1 Abth. i>. 169, Sitzungsb. d. K. K. Akad. d. 

 Wiss. Math. Nat. CI., Win,, [882. 



