254 



I8CHNOCHITOMN 1 -DINOPLAX. 



i, in which there is apparently no such relation 

 between slits and external ribs. 



This last subfamily includes the majority of species, and a consider- 

 able number of genera ; and while there are still some obscure points, 

 the light cast by what we already know of their morphology is suffi- 

 cient to show us the approximate paths of generic descent, the genera 

 falling into three main groups, as shown in the diagram. 



Ischnochitoninse 



Callistoplacinse 



It will be noticed that Leptoplax and Spongiochiton are not 

 included. These genera belong to the Acanthochitidaz. Moreover, it 

 must be remembered that Trachyradsia is a synonym of Stereochiton 

 (see p. 52) and part of Trachydermon belongs to the Tonicelloid 

 branch. These groups were erroneously placed in the text. 



The following genera belong to the Chudopleuroid branch of 

 Ischnochitoninae and should have been inserted after the genus 

 Chcetopleura (p. 27.) 



Genus XXL DINOPLAX Carpenter, 1882. 



Dinoplax CPR. MS., in DALL, Proc U. S. Nat. Mus. 1881, p. 284, 

 287 (Jan. 20, 1882). Acanthopleura (in part) GRAY. Chwtopleura 

 (part) Sur i n. I:\VMR MI. r/,,7o s. s. (in part) ADAMS, Gen. Rec. 

 Moll, i, p. 474, 475, not of Linn. 



Valves heavy, exposed, the lateral areas high ; teeth of insertion 

 sharp, smooth, those of the posterior valve directed forward; slits of 



