MOPALIID^E. 293 



Family MOPALIID.E Pilsbry. 

 Placiphoroidea (part) and Mopaloidea (part) CPU. 



Chitons in which the valves are divided in the normal manner 

 into lateral and central areas ; the teeth of insertion are not pectina- 

 ted ; and the posterior valve has a posterior-median sinus, with one 

 slit on each side or none. The girdle is more or less hairy, never 

 ;scaly. Gill row as long as the foot. 



The normal number of slits in the anterior valve is 8 ; but this is 

 often increased or diminished by splitting or fusion of one or two 

 teeth. The slits correspond in position to external ribs, as in the 

 last subfamily. In certain forms (Placiphorella, Placophoropsis) 

 the number of anterior slits is abnormally multiplied. The inter- 

 mediate and posterior valves have not more than 1 well developed 

 slit on each side, and even this is lacking in the tail valve of Plaxi- 

 phora. 



The girdle clings more tenaciously to the teeth than in most 

 chitons, being like that of Ceratozona. It is frequently provided 

 with pores at the sutures, each pore generally bearing two or several 

 bristles or hairs ; but the presence or absence of these pores is in no 

 case of more than specific value, and in some cases I have found it to 

 be a mutable feature among individuals of the same species. 



The present family is in no manner identical with that so named 

 by Dall, or with Carpenter's division of similar name ; nor is it in 

 any sense equal to the Plaeophoridce of Dall, Placiphoroidea Cpr. 

 These Carpenterian divisions of the " Irregular Chitons " are very 

 artificial, just as the primary division of the group into " Regular " 

 and " Irregular " forms is artificial. 



The ancestors of Mopaliidse were Callistoplacince, probably not 

 very different from the recent genus Ceratozona. 



Key to the genera of Mopaliidce. 



a. Posterior valve having a slit on each side of the median sinus. 

 b. Shell oblong ; girdle narrower in front than at the sides, 

 its hairs smooth ; teeth of insertion not lobed. MOPALIA. 

 bb. Shell rounded-oval ; girdle expanded and broadest in front, 

 its hairs scaly ; teeth of insertion more or less lobed. 



PLACIPHORELLA. 

 <ia. Posterior valve having a median tail-sinus but no slits. 



PLAXIPHORA. 



