192 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



specimens of Ph. imperalis. The crowding of the pores is very similar to what 

 occurs in Asterias and other starfishes, where the ambulacral plates are so crushed 

 together that a straight, single row of pores is forced into such a zigzag arrange- 

 ment that it has the appearance of two parallel series. There is no reason to con- 

 sider the arrangement in Diplocidaris as anything other than a highly specialized 

 condition. It seems strange that it is not found in any living species of Cidaridae. 

 Dbderlein ('87) lists 5 species of this genus, all from the Jurassic strata of 

 Europe. 



TETRACIDARIS. 



Tetracidaris Cotteau, 1872, Rev. et Mag. Zool. (2), 23, p. 445. 



Plate 29, figs. 7-11, Rev. et Mag. Zoiil- (2), 23, Cotteau, 1873. 



Test large, circular at ambitus, somewhat depressed ; coronal plates very numer- 

 ous (16 in each complete vertical series), arranged in 4 series in each interradius 

 from actinostome to above ambitus and thence in a double series to the abactinal 

 system ; areolae somewhat sunken ; median interambulacral areas narrow and with 

 few miliaries; ambulacra narrow, only about .20 of interambulacra in width ; porif- 

 erous zones little sunken ; median ambulacral area nearly bare, with a marginal 

 series of tubercles and a few scattered miliaries; pores nearly horizontal, widely 

 separated, and crowded into a double series in each zone, much as in Diplocidaris. 

 Abactinal system " large." Actinostome? Primary spines rather slender, nearly 

 cylindrical, somewhat ridged. Secondaries and pedicellariae 1 



In the arrangement of the pores this species is intermediate between Diplo- 

 cidaris and Phyllacanthus, but it is not in any sense a connecting link between 

 these genera. It may be regarded as a specialized offshoot of the Diplocidaris 

 branch. Duncan ('89) thinks it may be related to Astropyga, and there is some 

 reason for thinking it is not genetically connected with the Cidaridae at all. Only 

 one species is known, reynesi, from the European Cretaceous strata. 



STEPHANOCIDARIS. 

 Stephanocidaris A. Agassiz, 1863, Bull. M. C. Z., 1, p. 18. 



Test, ambulacra, interambulacra, and relative proportions as in Phyllacanthus, 

 but coronal plates 6-8; abactinal system .40-.45 h. d. and actinostome either larger 

 or smaller; anal system large and made up of numerous plates (in a specimen 

 42 mm. h. d. there are over 50 anal plates, and in a young specimen 12 mm. 

 h. d. there are 25) ; all plates of abactinal system relatively thin; genital plates 

 much wider than high, except madrepore, which is much larger than others ; 

 ocular plates wide and high, 4-sided, outer side convex, inner usually correspond- 

 ingly concave; genitals and oculars together form a ring around anal system of 



