PALEOPNEUSTES AND LINOPNEUSTES. 



185 



According to Jackson. 1 such wedge-shaped plates, which are little hy 

 little pushed back from the radial or interradial line of junction by the con- 

 stant movement of the plates of the test, are considered as being newly 

 intercalated plates in Melonites. Certainly in Linopneustes and Phrissocystis 

 their shape is due to the pressure of the coronal plates at the sharp angle of 

 the ambitus, both from the actinal and abactinal side, which prevents their 

 regular expansion. 



Fig. 275. Linopneustes longispinus. 



The apical systems of Linopneustes and of Paleopneustes, Figs. 260, 261, 

 differ but slightly. The genital plates in Linopneustes (PI. 94, fig. 7) are 

 much less elongated than those of Paleopneustes (PL 97, fig. .'), and in the 

 former the genital openings are closer together or vary in number as men- 

 tioned above. The ocular plates of Paleopneustes are also larger and more 

 triangular than those of Linopneustes. The primary radioles of Paleop- 

 neustes (PL 97, figs. 4> o) resemble greatly those of the regular Echinidse. 



1 Bull. Geol. Soc. Am. 1S9G, Vol. 7, pp. 152, 19(j. 



