240 AMERICAN JOURNAL 



from the drawing of Mr. A. Adams in the Gen. Rec. Moll., 

 appears to agree in all essential particulars with Acmcea. Still 

 it is not improbable that some of the species may prove to differ 

 sufficiently from the Acmgeas to retain a separate generic name. 

 The anterior margin of some of them is almost straight, and one 

 of these was described as a Crepidula by Prof. C. B. Adams. 



The statement made by Forbes, and copied by Gray, Jeffreys 

 and other authors, that the free branchial plume of Acmcea is 

 equivalent to the circle of leaflets around the mantle of Patella, 

 is manifestly erroneous. No one can examine the latter without 

 perceiving that the two organs are not homologous ; the lappets 

 of Patella rise in a different place from the cervical branchia of 

 Acma'a, they are supplied by different veins and nerves, and in 

 those forms where the circle is interrupted before the head the 

 ends of the cordon are far removed from the point of insertion 

 of the cervical branchia. Nothing more is needed to show the 

 erroneous nature of such a comparison after the discovery of 

 species in which both the cordon and the cervical plume are 

 found, as in Scurria mesoleuea. On the other hand, it is equally 

 true that the coi'don of Patella is not to be homologized with 

 the respiratory organs of the Chitoae. An examination of any 

 species of Chiton will show that the branchiae are, each in itself, 

 of a radically different construction from those of Patella, each 

 representing a plume furnished with transverse laminae, analo- 

 gous to tiie single plume o^ Acnuva. This conclusion is irresist- 

 ible upon a careful examination of the branchise by any one 

 possessed of a slight knowledge of comparative anatomy, and is 

 fully confirmed by Dr. Williams in his admirable paper on the 

 mechanism of aquatic respiration in invertebrate animals. 

 (Ann. Nat. Hist. 1855, p. 413.) The branchial system of the 

 Fissurellid<«,* according to that author, differs widely from that 

 of the Docoglossa, and the dentition and other anatomical details 

 confirm his conclusions from the study of the branchiae. The 

 gills of Emarginula and Propilidium offer closer analogies with 

 the Acmreid;>2, but other characters show that their strongest 

 affinities lie with the true Scutibranchs, though the dentition of 

 Propilidium remains to be examined. 



Blainville supposed that the function of respiration in Patella 

 was carried on especially by a net work of vessels in the thin 

 and delicate area of the mantle over the head, which I have 

 termed the "hood." It is by no means improbable that this is 

 the case in the Abra?ichiata, and to some extent the branchia in 

 the Patellida^ may also be assisted by the mantle, though this is 

 not yet proven. I have noticed in many species a most beauti- 

 ful net work of vessels in this locality, which cannot be without 

 an office of some kind. 



