found on the coast of Northumberland and of Durham. 239 



of Mtiller, an allied species, " inhabits the rocks of the sea, fixing 

 itself by a copper-coloured byssus." 



Crenella. nigra = Modiola nigra*. 



The specimens which I have got of this shell on our coasts are 

 very different in colour from those found in the Frith of Forth : 

 when Jths of an inch long they are of a brownish green colour ; 

 when an inch and three-quarters they are chestnut-brown ; an- 

 other difference obtains in the striae, which are much finer on the 

 Northumberland specimens than on those from the Forth. 



The generic name which has been given to the two last shells 

 requires a few observations. After examining the characters of the 

 species which served as the type of Capt. Brown's genus Crenella, 

 and comparing them with those of the so-called Modiola marmo- 

 rata, M. nigra, M. sulcata, &c., I have been led to the conclusion, 

 that these shells cannot be generically separated from Crenella 

 decussata. 



As regards external form, though the difference is great be- 

 tween Crenella decussata and C. nigra, yet how are we to distin- 

 guish the former from C. faba and C. [Modiola) glandula, Totten, 

 which run completely into the latter ? and as to the crenulated 

 hinge-plates of C. decussata, they are to be seen, though gene- 

 rally less developed, in all the species that have been quoted. 



In separating these shells from Modiola, I have been influenced 

 more by the example of others than by any opinion of my own. 

 Considering them as a single group, they have at various times 

 been differently named: it would appear from Swainson that 

 Humphreys distinguished it by the name of Lanistesf ; in the 

 ' Synopsis of the British Museum ' they appear to be named Mo- 

 diolarca ; Swainson calls them Brachydontes ; and Beck designates 

 them Modiolaria. Mr. J. E. Gray even goes so far as to make a 

 distinct family for them under the name of Crenellidce, which 

 <( differs from that of Mytilida (Mytilus, Modiola) in the mantle 

 lobes being united together so as to leave only two posterior holes 

 for the entrance and exit of the water, and a slit for the foot and 

 beard J." It is possible I am in error as to the species which 

 Mr. Gray places in the genus Modiolarca ; it may be mentioned 

 however, that in Crenella (Modiola) marmorata and C. (M.) nigra, 

 there are, as in Modiola vulgaris, a long slit and only one " pos- 

 terior hole ;" the latter for the egress current, and the former for 

 both the ingress current and the foot : in Crenella marmorata, 

 owing to the anterior adductor muscle being strap-shaped, and 

 extending unusually backward, the slit actually occupies the pos- 



* Vide Montagu's Supplement, pi. 26. fig. 4. 



f This name does not occur in Humphreys's Catalogue. 



X Synopsis of the British Museum. v 



