164 ANATOMY OF THE LAMELLIBRANCHIATA. 



Avicula longicostata. 



Shell inequilateral, with six raised casta radiating from the umbo, each 

 rib extending far beyond the margin of the valve; minutely striated between 

 the ribs. 



This very elegant fossil is remarkable from the great ex- 

 tension of the ribs which radiate from the umbo, in several 

 instances extending more than an inch beyond the margin of 

 the valve. In the five specimens which have come nnder my 

 observation, there appears to have been a determinate stop- 

 page of growth, which is evidenced by a raised line crossing 

 the disc of the shell and spinous terminations of the first- 

 formed ribs. 



The internal characters determine the genus to which it 

 belongs, while the external characters alone would have left 

 me in doubt to which of the following genera it most proba- 

 bly belonged ; viz. Avicula, Pecten, or Plagiostoma. The 

 number of ribs (six) appears to be a constant character, at 

 least judging from the specimens which have already come to 

 hand. 



For this beautiful fossil we are indebted to the excavations 

 made through the lias shales along the line of the great west- 

 ern railway, at Saltford, between Bristol and Bath. Avicula 

 Cygnipes, figured in Phillips's 'Geology of Yorkshire' resem- 

 bles it, but the two are sufficiently distinct to justify their 

 separation. 



Bristol, January 16th, 1839. 



Art. V. — On the Anatomy of the Lamellibranchiate Conchiferous 



Animals. By Robert Garner, Esq., F.L.S., 



( Continued from Page \29). 



DIGESTIVE SYSTEM. 



All these animals derive their nourishment from the ani- 

 malcules and other nutrient particles drawn in with the water, 

 by means of the currents excited by the ciliated branchiw and 

 tentacles. The particles are collected at the anterior part of 

 the cavity of the mantle, and are conveyed into the oesophagus 

 by means of tentacles and lips, strongly ciliated internally for 

 that purpose. In some species, as the Pecten, the foot seems 

 a prehensile organ of the food, and the curious foot of the 

 Spondylus is perhaps of some use in this way. The tentacles 

 are precisely similar to the branchice in structure, being 

 commonly membranous, striated by the vessels, and ciliated ; 

 and their vascular system is often continuous with that of the 

 branchice, so that they probably serve the purpose of respira- 

 tion also. They are generally triangular in shape. In the 

 Nucula, the external one is large and spiral ; in the Corbula 



