Dr. J. E. Gray on some Families of Bivalve Shells. 409 



I am not aware of any attempt ha\'ing been made to improve 

 the systematic distribution of Bivalve Molhisca foi-med on the 

 actual examination of the animals, or to embody the observations 

 which have been recently made by other naturalists, since my 

 arrangement of the families, published in the Synopsis to the 

 British Museum in the year 1840, and its revision in 1842. Yet, 

 during the last twelve or fourteen years, we have been furnished 

 with some most important observations on the subject by 

 Messrs. Alder, Hancock, Clark, and Edward Forbes in our own 

 country, and by M. Philippi, Deshayes, Valenciennes and others 

 on the continent of Europe. 



. I consider such a periodical revision of the labours of others, 

 especially when accompanied by personal obsenation, to be very 

 important, as bringing together, and putting into a connected 

 form and systematic order, the discoveries which have been made 

 in the meantime, or observations the importance of which may 

 have been overlooked by myself or others when preWously 

 occupied on the subject ; and they evidently have considerable 

 effect, as is shown by the complete manner in which my revision 

 of the system of Gasteropoda, in the fourth volume of the ' Figures 

 of Mollusca,' and the more recent improvements suggested in it, 

 have been adopted by Messrs. Adams, in their very useful work 

 on the ' Genera of ^loUusca,' and the partial adoption of it in 

 Phihppi^s ' Handbuch der Conchyliologie und Malacozoologie.' 



I now proceed to give the arrangement proposed. 



Class I. COXCHIFERA. 



Subclass 1. SiPHONOPHORA. Mantle leaves connected, with two 

 siphonal openings behind. 



In many of these animals, especially in those which have a 

 small pedal opening, there is a minute (4th) aperture with an 

 internal valvular protuberance in the mantle under the lower 

 siphon. 



The gills in most families hang down on each side of the foot. 



In Anatinida, SolenomyadtB, and Pandorida they have been 

 described by some authors as single, folded on itself, and by others 

 as the two gills soldered together; they appear to be attached 



la famille des Venus vers le milieu de la serie des MoUusques acephales 

 Dimyaires." The reply is easy. They are placed at the head of the class 

 for the ver)' reason M. Deshayes has assigned, that they api)eared to me 

 to be the most perfectly developed, and consequently the most typical animals 

 of the class ; and as it is our habit to place the Primates, hisessores and 

 Percida at the head of the respective classes of animals to which thev 

 belong, so it appears legitimate to place the family Venerida at the com- 

 mencement of the Bivalves, the other families diverging from it in two or 

 more series. 



