696 DEEP WATER MOLL USES AND BBACHIOFODS—DALL. vol.xvii. 



are iieitlier typically foliobrancliiate nor normally reticulate. Hitherto 

 those who would separate the fllibranchs ordinally from the modern 

 reticulate forms have been obliged to institute an intermediate order, 

 "pseudo lamellibranchs," to receive those pelecypods with a "fili- 

 brauchiate" organization which persist in developing reticulate gills. 

 The evidence of the ancestry of the tilibranchiate types afforded by 

 paleontology is sufficiently weighty to show how artificial is any such 

 arrangement, and how little in accord with thephylogeny of the forms 

 concerned. But while the transition between the tilibranchiate and 

 reticulate gills has always been sufficiently obvious, there has been a 

 very marked gaj) between the foliobranchs and any of the others. This 

 the present type does something to bridge, or, at least, to indicate how 

 it might be and probably was bridged in the past. It adds something 

 also to the testimony for archaicismin the Anomalodesmacea which the 

 present writer, in conformity with paleoutological evidence, has pointed 

 out. 



Mter the above was written the writer was unexpectedly enabled to 

 examine the gill in two species of Callocardia^ dredged in the Pacific 

 Ocean by the Albatross off' the coast of Central America, in about 400 

 fathoms. Contrary to the known Cardium-Uke type of reticulate gill 

 which characterizes the shallow water Isocardia (with which Callo- 

 cardia has hitherto been associated as a subgenus), the ctenidium 

 proves to be even nearer to the typical foliobranch gill (such as that 

 of Solemya) than is the gill of Euciroa. The single ctenidium in Callo- 

 cardia stearnsii, Dall, is composed of the central stem and two sets of 

 ribbon-like lamellie, which spriug from either side. These lamelliE 

 are thick and fleshy (relatively to their size), and are attached to each 

 other at their proximal ends by the common adhesion to the stem, and 

 at their distal ends by a narrow fibrous strip, which may possibly 

 contain a vascular channel, but did not show any in the present condi- 

 tion of the specimens. There are indications of a lateral band of 

 cilia; at all events, the edges of the lamelhe are distinctly marginate 

 and yet not organically connected. The inner limb of the ctenidium 

 is much the larger, rounded triangular in outline and with a bluntly 

 rounded keel below, the distal jiortion of the mass of lamelloe being 

 reflected and closely appressed to the direct limb, and reaching upward 

 about two-thirds of the way from the point of reflection to the arterial 

 stem. The outer limb is very much smaller than the inner one, but 

 has the reflected part longer and larger than the direct, so that the 

 dorsal edge of the reflected portion extends toward the middle line of 

 the body over the stem, covering the dorsal edge of the direct part. 

 (See figure 1, A, b.) 



The shell of Gallocardia closes so tightly that the preservative used 

 had penetrated slowly and the specimens are not in a condition to use 

 for sections. It can be positively stated, however, that there are neither 

 fibrous nor vascular connectives between tbe lamella^, excei)t as above 

 mentioned, and the chief difference betwoeu the ctenidium of Callo- 

 cardla and that of Solemya is obviously that the lamellae are united by 



