225 



Prof. Agassiz said that he had recently received from 

 Capt. Atwood of Provincetown, a specimen of the common 

 Porpoise of our waters. 



It had hitherto been considered as identical with the PhoccBna 

 communis of Europe, but his examination had led him to regard 

 it as a distinct and hitherto undescribed species. In P. com- 

 munis the temporal groove of the skull is narrow and oblong ; in 

 the American species it is as wide as long. The general form 

 of the skull is also diiferent. In the European species the pos- 

 terior surface is nearly vertical, in the American it is much 

 curved. The teeth of the American species, although agreeing 

 in general with those of the European in form, are grooved on 

 the broad faces near the summit so as nearly to divide them into 

 three lobes ; in the European they are smooth. The dorsal fin 

 is serrated and furnished with very characteristic tubercles in 

 the American species, which are not mentioned in the descrip- 

 tions of P. communis. Prof. Agassiz exhibited drawings of 

 the external appearance and of the muscular structure employed 

 in the movements of the tail. He proposed for the new species 

 the name P. Americana. 



Prof. Agassiz said, that at a former meeting he had 

 spoken of the connection of the gills of Crustacea with the 

 legs, and he now proposed to give the result of his examin- 

 ation of the solid framework of those animals with refer- 

 ence to these organs. 



Milne Edwards, he remarked, only notices those parts which 

 are external to the shield, thus omitting structures of great im- 

 portance. On cutting across the shell of a Lobster, it is found 

 that the membrane lining it is reflected at the lower margin, and 

 forms a sac containing the gills, not communicating with the 

 proper cavity of the shell, but opening outwards for the admission 

 of water. Prof. Agassiz was at once struck with the analogy which 

 this fact presented, and for which he had long been seeking, 

 between the gills of Crustacea and the tracheae of insects. Sup- 

 pose the sac of Crustacea to be divided transversely, making a 

 separate cavity for each gill, and the arrangement would be similar 

 to that in insects, in which each trachea is situated in a fold of 



PROCEEDINGS B. S. N. H. 15 MAY, 1850. 



