84 The Rev. R. Sheppard on tzvo new 



not notice in that of M. anatinus ; but the hinge in the first spe- 

 cies is scarcely more lateral than in the last, and not so much 

 as in my M. incrassatus. In the M. anatinus, he speaks of the 

 umbones as being decorticated ; but mentions not that circum- 

 stance in his description of the M. ci/gneus ; thereby leading us 

 to suppose that they are not so in that shell ; whereas, the four 

 species have their umbones decorticated, — in a greater degree, 

 indeed, in the M. anatinus than in the rest. 



With respect to the accompanying plate, indifferent as it is, 

 it may be useful for determining the species by bringing them 

 into one point of view. The outlines are the size of specimens 

 in my collection ; and the shells having been laid on the paper 

 and their circumferences taken, their exactness may be depended 

 upon. 



1. Mytilus cygsteus. 



M. testa ovata, anterius compressiuscula, fragilissima, margine 



membranaceo, umbonibus decorticatis, antica baseos adscen- 



dente. 



Tab. V. Fig. 3. 



Habitat in rivis et stagnis. 

 Long. 2 poll. 71 lin. Lat. 5 poll. 



On this species and M. anatinus I need not enlarge, they being 

 so well described by Dr. Maton and Mr. Rackett in their admi- 

 rable paper upon the British Testacea. I shall only observe, 

 that what was omitted by Linnaeus has been unnoticed by them, 

 viz. that the margin of M. cygneus is membranaceous, though 

 not in so great a degree as that of M. anatinus ; and that its um- 

 bones are decorticated. It arrives at a much greater size than 

 the one figured, or than the largest specimens of M. anatinus. 



2. Mytilus 



