236 



COMPARISON. 



C. reccns, measures 5X3 mm. C. johnstoni, 7X4^ mm. 



Whitish, with pale grey and brown Light buff, pinkish tinge, orange spots. 



flecks. 



Side slope straight. Side slope slightly curved. 



Girdle somewhat banded. Girdle banded. 



Lateral areas closely granulated, Lateral areas with two much raised, 



more so still on posterior edge. very distinct, radial ribs. 



In pleural area granules clearly In pleural area granules arranged in 



arranged in longitudinal rows, longitudinal rows, very closely 



not closely packed in the rows. packed, and becoming long and 



finger-like. 



In both, the slits in the anterior valve correspond with the ribs, the mucro 

 is shallow, and the posterior slope gradual in both. In C. rccens, nothing is 

 said about the scales, beyond that they are large and have 8-9 strong ribs, 

 whereas the larger shell, C. johnstoni, has 6-7 ribs. 



Pilsbry, in Man. Con., vol. xv., p. 260, places great emphasis on "the peculiar 

 insertion teeth, which are curved upwards into the ribs as if festooned," as 

 being one of the most important characteristics of his genus Callistochiton. Dr. 

 Thiele makes no reference to this feature in his C. recens, but we must presume 

 that he would hardly have placed it in that genus had such not been the case, 

 neither does he make any mention of radial ribs in the posterior valve, nor the 

 twin ribs of the lateral area. Further, he makes no mention of the protuberances 

 I have called teeth, on the posterior margins of the valves, and no mention is 

 made of the unique, finger-like, knobby granules of the pleural area and tail 

 valve, all of which are marked features in C. johnstoni. 



It is unfortunate that the differences have to be so largely determined upon 

 negative data, but there is one piece of positive data available. Thiele states (I.e.), 

 "We see the shell is like that described and drawn in Callistochiton finschi 

 (Zool. Heft., 56, p. 86, tab. 8, figs. 57-60)." A reference to these figures shows, 

 practically no correspondence between it and the shell from Carnarvon. 



For description of Plates see pp. 242 and 243. 



