60 EOCENE MOLLUSCA. 



fresh -water Limnaeae in company with salt-water animals, as previously noticed by Mr. 

 James Sowerby in his ' Mineral Conchology.' I am inclined to think the water where 

 these animals were deposited was quite fresh, although Limnaeae have been found living 

 in saline marshes; the reptile was probably stopped by a bend of the river or by 

 some impediment in the stream; the presence of a large number of the scales of 

 Lepidosteus in this layer of sand indicates the greater probability of its having been more 

 of a fresh-water deposit than brackish. 



D. Brardii is given by Mr. Prestwich in his list of the organic remains of the 

 Woolwich and Reading series (Jour. Geol. Soc, 1847, p. 117); he says, however, at 

 p. 120 (/) — "this shell is here in too imperfect a state to admit of a positive 

 determination." 



Two specimens, with the name of Mytilus Brardii, were obligingly sent to me 

 many years ago by the Comte du Chastel, and to these was attached the locality of 

 Bordeaux. I presume they are what have since been called Dreissena Basterotti, 

 On a close comparison with the British fossil, these specimens present the following- 

 differences : they are rather more pointed, less curved at the umbo, and they have an 

 obtuse elevation or sort of ridge at the most inflated part, which has probably caused 

 them to be called " subcarinated." Whether these slight differences be permanent or 

 whether they are such as will constitute a specific distinction, the few specimens of the 

 Bordeaux shell that I have seen will not permit me to give a decided opinion. 



MYTILUS. Linnaus, 1758. 



Generic Character. Shell equivalve, wedge-shaped, acuminated, rounded on the 

 siphonal side ; umbones pointed, terminal ; hinge-teeth few, minute, sometimes obsolete ; 

 muscular impression at the umbo small and narrow ; pedal impressions two in each 

 valve ; connexus bipartite, marginal ; interior of shell nacreous. 



Animal elongate, with the lobes of the mantle partly fringed, plain in the anal region, 

 and slightly projecting ; disconnected except where there is a separation for the siphonal 

 openings ; adductor muscles very unequal in size ; foot cylindrical, furnished with a gland 

 and groove; byssus strong and coarse. 



This genus is generally marine, though sometimes estuary in its habits, and, I believe, 

 M. edulis is known occasionally to live with Limnaeae, &c, when the water is fresh. This 

 common edible mussel frequents mud banks, and, as it is well known, is more often found 

 where it has been deserted by the retiring tide ; others inhabit the sea at considerable 

 depths. In the recent state the genus numbers probably fifty species, and these have a 

 very extensive geographical range, taking in the whole circumference of the globe. The 

 generality of the species are extra-tropical. Fossil species are abundant, and are said to 

 be found in rocks of the Permian age. 



