PLAXIPHORA. 315 



England. (U. S. Fish Commission Steamer 'Albatross', 1883.) 



Placophora (Euplacophora) atlantica V. & S., Amer. Journ. of 

 Science xxiv, Nov., 1882, p. 365 (footnote) ; Trans, of the Connecti- 

 cut Academy of Natural Sciences, vi, p. 206, t. 30, f. 1, la, 16. (May, 

 1884). Placophora atlantica V. & S., BALL, Bull. 37, U. S. Nat. 

 Mus., p. 174, t. 45, f. la, b; t. 63, f. 102, 102a (reproduced from 

 Verrill & Smith.) 



A very peculiar species. The figures of the tail and head plates 

 given by Verrill and copied on plate 66 are not very good, and I 

 have therefore drawn others from one of the original specimens, on 

 pi. 65. 



The only specimen of this species which I have examined has a 

 deep cleft in the posterior margin of the tegmentum of the tail valve, 

 parallel to the lower border of the valve. Whether this is or is not 

 abnormal must be decided by the examination of more material. 

 It is well shown in fig. 74, of plate 65. 



Subgenus PLAXIPHORA Gray. 

 Under this subgenus three sections, defined above, are placed. 



Section Plaxiphora Gray. 



This section as here restricted corresponds to Gray's original 

 genus, except that he was ignorant of the fact that sutuial pores or 

 tufts are absent in some species. It is absolutely equivalent to 

 Shuttleworth's section Euplaxiphora, but Carpenter restricted this 

 last name to species of the P. petholata type, having no pores or 

 tufts at the sutures. The mutability of this character has been dis- 

 cussed under Mopalia; therefore it is necessary in this place merely 

 to state that the presence of pores at the sutures is not even a con- 

 stant specific character in Plaxiphora, except when the tufts are 

 strongly developed. There are several species in which the pores 

 are visible in some specimens, but quite absent in others preserved 

 in the same way. 



The corrugated Plaxiphoras are in great need of revision ; but the 

 material now in museums is insufficient. Great latitude may be 

 expected in the range of specific variation, as in the genus Mopalia* 



Partial key to species of Plaxiphora. 

 a. Central areas unsculptured save for growth-lines. 



