INSTITUTE OF SCIENCE, PHILADELPHIA. 55! 



incut not divided, the teeth are of the same character. Many Pelecypods 

 having no intimate relations to hocardia have been referred to this family by 

 Paleontologists, on the ground that they have gyrate umbones. Most of these 

 belong to the Veniellida and other diverse groups. 



FAMILY CALLOCARDIIDyE. 



Gills snbfoliobranchiate, with an inner and outer direct and reflected lami- 

 na, with a non-reticulate structure of fleshy transverse lamellae, ribbon-like, 

 united by their insertion at the stem and, distally, by a fibrous band connecting 

 the tips of the lamellae and making an edging to the lamina as a whole ; foot 

 short, with a flattish sole in front, behind produced and rounded ; with a byssog- 

 enous gland but no byssus; pallial line more or less sinuated ; margin of the 

 mantle more or less papillose, with developed separate siphons, otherwise as 

 in Isocardiida ; abyssal, marine. 



Shell as in Isocardiidce, but with low and inconspicuous beaks, the valves 



more elongated, and the lunule delimited by a sharp groove; cardinal formula 



I. o i o : a i 



It 101010- 



Recent, in deep sea ; probably also Tertiary. 

 Ex. Callocardia, Callogonia. 



In Callogonia there is a deep sinus in the pallial line, and a distinct anteri- 

 or lateral close to the cardinal teeth. The structure of the gill in these abyssal 

 forms is totally different from that of the shallow-water Isocardiidce. It is pos- 

 sible that the subfoliobranchiate type may be found to occur in other abyssal 

 forms, since the gill of the abyssal Euciroa is in many respects analogous. 



D. Teleodonta. 



The most perfected type of modern teeth, to which, in addition to the nor- 

 mal (10101) cardinal series of the ordinary Teleodesmacea, is added, in the 

 most specialized types ( Venerida, Mactridce), either a roughened area (Venus), 

 a series of extra cardinals (Tivela), or accessory lamellae (Mactra), rendering 

 the hinge more efficient, or complicated. The hinge characters of the less 

 specialized forms hardly differ from the Diogenodonta, but they are grouped 

 here on account of their obvious affinities, as shown by other characters. 



Superfamily VENERACEA. 



Teleodonts with normal gills united to form a complete anal chamber, the 

 mantle lobes free behind the siphonal region, subsqual adductors, an external 

 parivincular ligament seated in a groove, and the shell substance densely cellulo- 

 crystalline with inconspicuous epidermis. The aorta has frequently a post- 

 ventricular dilation. Complete hinge formula jj \ \ x o x o To i olhr!' ^ which a 

 large part is usually deficient. 



