1894. PROCEEDTNGS OF THE NATIONAL MUSEUM. 705 



attached to the mantle in the ordinary way. It is hardly credible that 

 he could have overlooked so prominent a feature, but there is nothing- 

 resembling it described by him. He figures au oval glandular spot on 

 the mantle, of which he says, " There is on the mantle a glandular 

 swelling comparable in its jjositiou to the hypobranchial gland of gas- 

 tropods." But this statement in no way expresses the condition or 

 relation of the parts in the j^resent species, or in Halicardia, and hence 

 we must suppose, if reliance is to be placed on Pelseneer's account, that 

 the species he examined differs from L. aJasJcana and Halicardia in 

 wanting the free lamina to which in these species the outer edges of 

 the gills are attached, and in having the gills attached directly to the 

 mantle. 



The lithodesma of the very young Halicardia is shown by a specimen in 

 the National Museum to be shaped like that of L. alasJcana and L. papyra- 

 cea, but in the adult Halicardia it has assumed a totally different form. 

 The character of the branchial siphon, pedal oi)eniug, lithodesma, and 

 details of the shell are sufficient to separate Halicardia from the LyonsicUa 

 of the type of L. alashana and, if we accept Pelseneer's account, the 

 latter can not be united with L. abyssicola, Sars, which is the type of the 

 genus Lyonsiella^hnt must be separated to form a separate groui), which 

 might be placed as a subgenus under Halicardia. But I must confess 

 to doubts as to Pelseneer's accuracy, in this i)articular,* sufticiently 

 strong to make me feel it inadvisable (until his account is confirmed by 

 new evidence) to name and separate the species allied to L. alaslcana. 

 In case they prove to agree with L. ahyssicola, Halicardia will have to 

 take its place as a subgenus under Lyonsiella as the older name. 



Genus PECTUNCULUS, Lamarck. 



PECTUNCULUS ARCODENTIENS, uew species. 



Plate XXVI, tig. 6. 



Shell small, rather inflated, thin, high, and sculptured with about six- 

 teen rounded, prominent ribs, with very narrow interspaces crossed by 

 fine elevated threads; area siiuill, wide, subtriangular ; hinge line nar- 

 row, evenly arched with about eight teeth on each side of the beaks; 

 basal margin narrow, indented by the sculpture, with obscure interlock- 

 ing dentations on the inner face opposite the interspaces between the 

 ribs; adductor scars distinct, on a slight raised area extending into the 

 umbonal cavity. Height of shell, 21.5; breadth, 20; diameter, 13 mm. 



Station 3172, in 295 fathoms. No. 107014, U.S.N.M. 



Although the single valve obtained is dead and has lost its color, and 

 the surface is somewhat eroded, yet its characters will not permit us to 

 refer it to any described species. None of the coarsely ribbed species 

 combine transverse reticulation with so thin and rounded a shell, and 

 it is quite peculiar in the evenly- rounded arch of its hinge plate. 



* Pelseneer has since admitted the incorrectness of his first account of the attach- 

 ments of the gill in Lyonsiella. Compare Arch, de Biol, si, 1891, p. 215, foot note 5. 

 Proc. N. M. 94 45 



