312 PLAXIPHORA. 



unslit, interrupted behind by a median, rounded sinus, which does 

 not generally emarginate the tegmentum ; mucro posterior. Girdle 

 bearing simple bristles or hairs, scattered or gathered into pores or 

 tufts at the sutures. Type Chiton setiger King. 



This genus is most nearly allied to Mopalia and Placiphorella, from 

 both of which it differs in the unslit insertion-plate of the tail valve. 

 The peculiar girdle-bristles of Pladphorella are an additional char- 

 acter separating the genera. 



In Plaxiphora, as in Mopalia and Placiphorella, the bristles of the 

 girdle are indifferently scattered or gathered into sutural bunches. 

 This character is of specific value only when strongly developed ; 

 and each of the three genera contain species which in occasional 

 individuals develop pores, or species usually poriferous, but varying 

 to poreless. This distinction, which Blainville, Gray and the Adams 

 brothers considered fundamental, is therefore of comparatively tri- 

 vial importance. 



Plaxiphora is divisible into a number of minor groups, as follows : 



Subgenus PLAXIPHORA (restricted.) 



Slits of anterior valve normally 8 (sometimes 7 or 9 by coales- 

 cence or splitting), coresponding in position to external ribs ; the 

 teeth long outside. Girdle widest at the sides. Mantle not fringed 

 in front of the head. Distribution, Southern extremities of the 

 southern continents ; living in the littoral zone. 



This subgenus although rather homogeneous, may conveniently be 

 divided into three sections, as follows : 

 Section I. Plaxiphora, restricted (=Euplacipliora Shuttl. not Cpr.). 



Valves exposed. Entire shell normal in form ; posterior valve 



not greatly reduced in size or altered in form. 

 Section II. Guildingia Cpr. Valves partially immersed in the 



girdle, which encroaches at the sutures. 

 Section III. Fremblya H. Ad. Valves exposed. Shell short and 



broad ; the posterior valve reduced to a narrow crescentic form, 



and seen to be strongly arched upward if viewed from behind. 



Subgenus PLACOPHOROPSIS Pilsbry. 



Slits of anterior valve numerous (about 20) ; teeth very short. 

 Girdle widest and much dilated in front. Mantle produced into 

 several long finger-like processes in front of the head. Distribution, 

 Northwest Atlantic ; living in deep water. 



