1894. PEOCKEDINGS OF THE NATIONAL MUSEUM. 697 



a narrow band distally and reflected in the former, while they are com- 

 l)aratively free and not reflected in the hitter. It may be added that 

 the entire ctenidium is solid and fleshy as in Xueula, and when lifted 

 separates from the body in a single mass. The two ctenidia are united 

 to the siphoual septum behind the foot, bat not to each other, so there 

 is no complete anal chamber. The palps in Callocardla are very small, 

 the fo )t is flattened and solelike below anteriorly and rounded behind. 

 The si])hous are complete and papillose, longer than in Isocanlia, but 

 still short. 



The discovery of this type may be said to iiractically complete the 

 series uniting the foliobrauch with the reticulate gill and give the quietus 

 to the classification based solely on the divergencies of the ctenidia. 



It can hard Ij^ be doubted that the gills of ^Jtciroa are, represented 

 by the degenerate small gill of Verticordia acuticostata, as formerly 

 described by me. It seems possible, as will appear under the next 

 species, that the fleshy septum of the so-called Septibranchia may be 

 partly a modification of such an inwardly-directed lamina of the mantle 

 as in Euciroa lies below the visceral mass; and in Halieardia has free 

 edges; which in Euciroa is merely connective, but in Ualicardia con- 

 tains an extension of the nepliridia. What i)art in Verticordia the 

 siphonal septum plays remains to be decided by further researches, 

 but it also contributes more or less extensively to the total septum. 

 Geologically the Verticordiidce are an ancient group, and the fossil 

 Pecchiolia would seem to be very nearly related to Euciroa. At all 

 events the latter, in its dorsal heart not pierced by the rectum, its single 

 aorta, and archaic type of gills, adds c: very interesting member to the 

 small list of pelecypods of varying aftinity, which retain in their organ- 

 ization indisputable traces of archaic origin. 



In this connection I was led to examine the following species, also 

 di edged by the Albatross, but on the northeastern coast of the United 

 States, a specimen of which recently came into my possession. Owing 

 to its large size the characters of this mollusk are very plain and 

 unmistakable. Hoping to obtain some light on the vexed question of 

 the origin of the fleshy septum of Verticordia, I examined it with a 

 good deal of interest, and found, as will be seen, a type of septum 

 which seems wholly distinct from either of those hitherto described. 

 The species referred by its describer to MyfiliiHcria and b^" the writer 

 to Verticordia s. s., must evidently form the type of a new genus. 



Genus HALICAEDIA, Ball. 

 HALICARDIA FLEXUOSA, Verrill and Smith. 

 Plate XXIII, figs. 1, 3, .5, 6; plate xxiv, fig. 3. 

 MytUhneria flexuosa, Vehkill and Smith, Trans. Conn. Acad., v., 567, pi. 58, fig. 

 38; Am. Joiirn. Sci., xxii, 1881, p. 302; Trans. Conn. Acad,, vi, 1884, p. 2.58. 

 Verticordia flexuosa, Dall, Blake Pelecypoda, Bull. Mus. Comp. Zool., xii, p. 286, 

 Sept., 1886. 

 The specimen was dredged east of Georges Bank, in the Gulf of 

 Maine, in G77 fathoms, brown sand; bottom temperature, 39° F., by the 



