648 



PERISCHOECHIXIDAE. 



How far the two families established b}' McCoy, which are founded almost 

 entirely upon the nature of the spines and their articulations, are warranted, 

 cannot be ascertained at present ; the presence of spines of a uniform struc- 

 ture is an important difference, as compared to spines of two different kinds ; 

 but the perforate or imperforate character of the tubercles is not one upon 

 which family characters could be established, unless accompanied by other 

 features. 



In Hall's genus Lepidechinus the ambulacra and interambulacra lap in op- 

 posite directions. Interambulacra in eight rows ; outer pentagonal, others hex- 

 agonal. Hall, in his Report on the Geology of Iowa, has given us excellent 

 figures of the tubercles, spines, and portions of the ambulacra! and interam- 

 bulacral plates {PI. XXVI). From all these figures it is apparent that the 

 pores of the ambulacral system are pierced directly through the plates in all 

 the genera. 



In Archaeocidaria (Fig. 2) there are four plates in the ambulacra; each 

 plate forming vertical rows, much as in Hipponoe and Holopneustes ; four 



Fig. 7. 



Fig. 2. 



rows of hexagonal and pentagonal interambulacral plates, each carrying one 

 large spine and a scrobicular circle. 



Fig. 2. Melonites multipora (two diameters). Diagram showing the number and arrangement of the 

 ambulacral pieces and pores, and connection of the former, with the first range of the interambulacral 

 pieces on each side, near the middle of the ambulacral area. 



Fig. 7. Palaechinus burlingtoniensis (about two diameters) Diagram showina! a part of two ambu- 

 lacra, and the intervening interambulacral pieces above the middle of the body. 



Fig. 4. Lepidesthes Coreyi (diagram enlarged three diameters), showing the number and arrangement 



