410 CRETACEOUS PELECYPODA 



X. Order,— OSTREACEA. 



The principal characters of the species belonging to this order consist in 

 the presence of a single, (the posterior), adductor, in the entirely disunited, cirrated, 

 mantle margins, internally sometimes provided with ocelli, and in the small size 

 of the foot. The gills are pectinate, one pair on either side, curyed, but usually 

 free, posteriorly ; the palps are also two in number on each side, united at the base 

 across the front ; the pedal muscles are very slightly developed or obsolete. 



The shells are of various shapes, generally somewhat depressed, equivalve or 

 inequivalve, free or attached, in the former case the valves being usually eared at 

 the side of the beaks, while in the latter the ears do not become distinctly developed 

 from the body of the shell ; hinge generally edentulous, with an internal or sub- 

 internal ligament ; adductor impression single, sub-central, generally more or less 

 completely united with the posterior pedal scar. The outer layers of the shell 

 are laminar, often largely cellular or cavernous ; the inner are porcellanous. 



The OSTREACEA are, as an order, the only true monomyarian Pelecypoda, 

 and this character, combined with the absence of any siphonal openings, inhalant or 

 exhalant, the small size, or total obliteration, of the foot, aa well as the not 

 uncommon sessile habitat, place the order lowest in the system of the Pelecypoda. 

 I have examined a few species of each family, and never found any distinct indi- 

 cation of the anterior adductor. The single one present corresponds solely to 

 the posterior adductor of other Pelecypoda, and cannot, therefore, be taken for a 

 compound of the two, as is occasionally stated in conchological works. The 

 mistake appears to have arisen from the small impression which the pedal muscle 

 sometimes produces; it is only traceable in those species which spin a perma- 

 nent byssus. The small development of the pedal muscle is a striking peculiarity 

 in the anatomy of the OSTREACEA, as compared with the MYTILACEA. 

 In the former, there is only a single pedal muscle present, becoming confluent 

 with the upper part of the adductor in the free, not byssiferous, species ; in the 

 free byssiferous species it divides into two very short branches at the posterior 

 end, and is attached above and quite close to the adductor ; in the sessile forms it 

 becomes quite obsolete. The anterior pair of pedal muscles is never developed ; 

 there is, however, occasionally a small muscle present attaching the posterior 

 end of the palps and the base of the branchia next to the adductor. The sexes 

 are united in some, distinct in others. The mantle often forms duplicatures on 

 the internal side, apparently for the purpose of hatching eggs. 



The order can be conveniently sub-divided into six families : Radtjlibm, 

 Fectinid^, Sfondtlibje, O&TBTiiBM, Rlacjjnidm, and Anomiibm. Of these, the 

 Rabtjlibm may be considered as the highest family, because they are always free 

 and equivalve. 



In a geological point of view, the OSTREACEA are a very important 

 group of Pelecypoda, a great many species being recorded among the fossils 

 characteristic of various formations; and taking the order as a whole, by far 

 the greater number of species is extinct, as might be expected. When we estimate 



