Scientific Reviews. 187 



Neither do the elevations always afford good distinctive charac- 

 ters, as they vary greatly in individuals. 



The epidermal colours of the Naiades are very limited. " The 

 ground varies from deep fuscous or black to pale yellow, frequejit- 

 ly passing through obscure green." 



The form of the umbones, or beaks, he does not consider as afford- 

 ing a permanent character, many of the species having them en- 

 tirely decorticated, and some partially so. In short, few are free 

 of this denudation. He, however, recommends the strictest atten- 

 tion to the situation of the beaks, whether they are medial, termi- 

 nal, &c. 



" The margins," he says, " should have our attention in exa- 

 mining a species." But we differ from the author in this matter, 

 and are persuaded that little reliance can be placed on these parts, 

 they being exceedingly variable in shells of the same species. 



" The muscular impressions are more important, and should always 

 have our attention in examining a specimen. But even this cha- 

 racter is not infallible. It should be understood that the animals 

 of this family always possess two pairs of muscles, used for locomo* 

 tion, or placed near or in contact with the two adductor muscles, 

 used solely for closing the valves. In the anterior margin they 

 are generally separate, in the posterior more generally confluent ; 

 but in the same species we sometimes find individuals presenting 

 two, sometimes four cicatrices, besides those of the cavities of the 

 beaks, and this depends in a great measure on the thickness of the 

 shell. If the specimen be ponderous, we often find the posterior 

 muscle of the foot attached to the side of the lamellar tooth, near 

 to its termination ; if it be thin, although of the same species, it 

 will be found generally confluent, or near to the great posterior 

 muscle. The cicatrices made by the attachment of the superior 

 part of the mantle in ponderous shells, generally will be found on 

 the under part of the cardinal tooth. Sometimes six or eight may 

 be found, and their direction is towards the lamellar tooth. In 

 thin shells, these cicatricss will be found in the cavity of the beaks, 

 generally traversing it in an oblique direction." 



These are important facts, and if they generally obtain, as Mr. 

 Lea asserts, they prove how extremely difficult it must be to ascer- 

 tain the species of this genus ; for we had placed much reliance on 

 these as distinctive characters. In the many marine shells which 

 we have examined, we have always found the muscular impressions 

 to afford unvarying and important characters. 



Our author further asserts that little reliance can be placed on 

 the ligament as a specific character. 



He finds that there are 41 distinct and well characterized species 

 of Unio, including the 18 new species which he has himself de- 

 scribed. The other alleged species he discards as mere varieties. 

 Of the Symphynota he describes 9 species. 



This paper is written with much care and discrimination, and 

 forms an important accession to our conchological information. It 



