302 MOP ALIA. 



threads radiating from the beaks. Anterior valve having 8 strong, 

 narrow, raised ribs, with one wider rib at each sutural edge. 

 Posterior valve depressed, the mucro situated at the posterior end. 



Interior white ; sinus very small and shallow. Anterior valve 

 having 8, median 1 slit ; teeth but little thickened along the slits. 

 Posterior valve having a rather wide moderately deep rounded tail 

 sinus, and a single slit on each side. 



Girdle leathery, dusty, with a hair-pore at each suture, and 

 some scattered or alternating hairs. 



Length 10, breadth 6 mill. ; divergence 95. 



Puget Sound (Kennerley) ; Sta. Barbara (Cooper). 



fMopalia imporcata CPU., Proc. Acad. Nat. Sci. Phila., 1865, p. 

 59. Placiphorella imporcata CPR., in Dall, Proc. U. S. Nat. Mus. 

 1878, p. BQQ. Osteochiton imporcatus DALL, 1. c. 1886, p. 211. 



This species closely resembles M. sinuata in size and sculpture. 

 The two species differ in the following details : (a) M. imporcata is 

 unicolored, while the other is elegantly marbled with blue and 

 tawny ; (6) imporcata has the sutural ribs more developed and very 

 distinctly serrated, whilst in sinuata the serration is hardly appar- 

 ent ; (c) imporcata has the longitudinal riblets of the central areas 

 larger and the radiating threads which lattice their interstices much 

 smaller and much closer ; and finally, (d) the tail valve in imporcata 

 has an entirely posterior mucro, and a much shallower posterior 

 sinus. 



The girdles are the same in both species, being leathery, generally 

 " dusty " in appearance in dried specimens, although scales are 

 quite absent. 



This species might be considered a subspecies of M. muscosa were 

 it not that it is smaller and more highly angled, of a uniform light 

 color, and has the mucro farther to the rear. The girdle-pores at 

 the sutures, and the entire sculpture are exactly like the pore-bear- 

 ing variety of muscosa which I have above described from Bolinas 

 Bay. It is very likely that the intervening links required to com- 

 plete this chain of forms will be found. 



The description and figures are drawn from Carpenter's type 

 specimen, now in the U. S. National Museum. 



The length is estimated in this and the following species, as the 

 specimens are somewhat curled. 



