188 



in every one of the several hundred new species since pubUshed by 

 me. 



In a subsequent paper, which I read before this Society in March, 

 1829, and which the Society did me the favour to have published in 

 its Transactions, Vol. III., new series, page 414, is a paragraph on 

 "Muscular Impressions.^'' I mentioned their importance, and stated 

 that they "should always have our attention in examining a speci- 

 men." And further, I stated that " it should be understood that the 

 animals of this family always possess two pairs of muscles, used for 

 locomotion, and placed near or in contact with the two adductor mus- 

 cles, used solely for closing the valves. In the anterior margin these 

 are generally separate; in the posterior, more generally confluent; 

 but in the same species we sometimes find individuals presenting two, 

 sometimes three, and sometimes four cicatrices, besides those of the 

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

 thickness of the shell. If the species be ponderous, we often find the 

 posterior muscle of the foot attached to the inside of the lamellar 

 tooth, near to its termination; if it be thin, altiiough of the same spe- 

 cies, it will be found generally confluent, or near to the great poste- 

 rior muscle. The cicatrices made by the superior part of the mantle 

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

 cardinal tooth. In thin shells these cicatrices will be found in the 

 cavity of the beaks, generally traversing it in an oblique direction." 

 Again in a paper read May 7, 1830, Vol. IV. page 67, in a note on 

 Hyria avicularis, I mention having " discovered that the extensor 

 muscle of the foot is attached to the internal base of the cardinal 

 tooth, and there forms a remarkable cicatrix," &c. In a subsequent 

 paper, when describing U7iio Browianus (a Hyria), I mention that 

 the "cicatrix of the extensor muscle is placed over that of the ante- 

 rior adductor muscle." 



As other "new characters," Prof. Agassiz states, that " in some of 

 the Naiades, the posterior portion of the gills only is found to be dis- 

 tended with eggs at the breeding season; in others the whole gill is 

 so distended." In a paper read to this Society in July, 1837, and 

 published in Vol. VI. of Transactions, page 48, new series, I men- 

 tioned, that "believing the oviducts would present to us the means of 

 discrimination in some of the species, having found them to be so 

 very different in the Unio irroratus, my attention had been particu- 

 larly addressed to these organs, in the few and small species of our 

 vicinity." In this paper I gave figures of four species, displaying the 

 position of the oviducts, two of them, the Unio ochraceus and the Unio 



