Anatomy of Cydostoma elegans. 279 



The/oo«isanoval disk, obtuse before and behind, reticulated above, 

 with roundish reticulations, its edge smooth ; its frontal margin without 

 any groove. 



Tentacula two, subulate, somewhat compressed, transversely annu- 

 lated, their apices somewhat inflated, obtuse, opake. 



Eyes placed at the thickened external base of the tentacula. 



Muzzle proboscidiform, notched in front, with a narrow border, trans- 

 versely annulated, having the mouth in its disk. 



Mantle^ not united to the neck of the animal, but perfectly free, as in 

 the Pectinibranchia, its edge crenulated. 



Operculum ovate, spiral, calcareous. 



When the animal is taken out of the shell the different internal parts 

 may be pretty distinctly traced through the outward integuments ; as at 

 fig. 1, where a male individual is represented, the penis, sac of visco- 

 sity, pericardium, intestine, stomach, lobes of liver and testicle are rea- 

 dily discernible. 



If an incision be made along the left side through the muscular pro- 

 cess by which the animal is attached to the shell, so as to expose the 

 branchial cavity, the vena cava will be seen running on each side ; on 

 the right, near to and parallel with the muscle and rectum ; on the left, 

 between the muscle and sac of viscosity : these are met on each side by a 

 vein running parallel with the edge of the mantle, and then become ob- 

 solete : from these the venous blood is carried through the ramified pul- 

 monary veins, which are spread over the upper surface of the cavity, and 

 all tend to a common trunk, by which the aerated blood is carried into 

 the auricle. This is nearly of the same size with the ventricle, but of a 

 paler color ; its irmer surface clothed with an intricate web of anastomos- 

 ing fibres ; at the orifice of the ventricle is a bilobed valve opening in- 

 wards : the inner surface of the ventricle is also furnished with fibrous 

 cords, but these are not so intricate, and are in most cases nearly perpen- 

 dicular to its surface. The pericardium is situated on the left side, below 

 the sac of viscosity, which is dark, with a minutely lobed exterior, and 

 inwardly composed of small intricate plates. 



The mouth consists of an oblong fleshy mass, somewhat bilobed be- 

 hind, from the centre of which above proceeds the oesophagus, and 

 beneath this the tongue, which is flat, ligulate, obtuse at the apex, and 



