ACANTHOPLEURA. 217 



Key to subgenera and species of Acanthopleura. 



A. Insertion-plate of tail-valve long, crenulated outside, and 

 with a median slit behind, other slits subobsolete or wanting ; 

 girdle beset with spike-like spines ; sinus denticulate ; inte- 

 rior purple-brown in the cavity and transversely engraved ; 

 the sutural-plates white, (Mesotomura) , A. echinatwn. 



A A. Insertion-plate of tail-valve long, crenulated outside, and cut 

 by numerous slits ; median valves much wider across the 

 sutural-plates than across'the tegmentum or outer coat of the 

 valve ; interior not engraved (Acanthoplenra s. s.) 



a. Interior purple in the cavity, sutural-plates white ; 

 median valves occasionally Radsioid ; sinus of second 

 valve feebly denticulate ; girdle broad, with long black 

 spines ; exterior black, A. spinosa. 



aa. Interior livid-flesh or livid purple-colored ; median 

 valves 1-slit ; sinus smooth ; girdle wide, with numer- 

 ous spinules; exterior dull, generally eroded, 



A. spinigera. 



A A A. Insertion-plate of tail-valve short, blunt ; median valves as 

 wide or wider measured across the tegmentum than across 

 the sutural-plates ; sinus smooth. 



a. Insertion-plate of tail- valve cut into teeth by 7-11 well- 

 developed, subequal slits (Maugeria.} 



b. Interior sea-green (intense or faded), with a 

 black tract behind the sinus ; spines of girdle 

 very small, valves generally eroded, 



A. granulata. 



aa. Insertion-plate of tail valve crenulated, having an 

 oblique slit on each side, the ridge between them either 

 continuous or unevenly serrated (AmphitomuraS) 



b. Median portion of tail insertion-plate irregularly 

 serrated, but without true slits like the lateral 

 tail slits ; cavity of valves purple-black, sutural- 

 plates white ; valves usually not eroded outside, 

 uniform black, granulated at the sides ; girdle- 

 spines rigid, black, tipped with buff, 



A. brevispinosa. 



