LAMELLIBRANCHIATA. 693 



pallial line being "entire" — viz., the Aviculidce and Myiilida (= the 

 Heteromyaria of Bronn). 



(3.) The following families are " homomyary," and have an "entire" 

 pallial line, the animal being in some cases devoid of siphons, while in 

 other cases short, non-retractile siphons are present — viz., the Arridce, 

 NuciriidcE, Modiolopsidcc, Trigoniidcz, Union idee, Cardinidce, Carditidcs, 

 Grammy siidcE, Astartida>, Crassatellidce, Erycinidtz, GaleommidcE, Tri- 

 dacnidce, Cardiida, Litnnlicardiidce, Chamidce, Rudistcs, Megalodontidce, 

 CyprinidcB, Ungulinidce, UnicardiidcE, and Tancrediidce. 



(4.) The following families are " homomyary," and are provided with 

 retractile siphons, the pallial line thus becoming " siriuated "— viz., the 

 VeneridcE, Do?iacid<z, PsammobiidcE, SolmidcE, Mact?-idce, Myidcs, Glyci- 

 ?nerido?, Gastrochcenidce, PholadidcE, Tereainidce, Lucmidce, Tellinidce, 

 Scrobiculariidce, Solemyidcz, Arcomyidce, A?iatinid<z, Pra cardiida (?), 

 Pholadomyidtz, and Clavagellidce. 



As regards their general distribution in time, the Lamellibranchs 

 are a very ancient group, the earliest representatives of the class 

 {Glyptarca, &c.) being found in the Upper Cambrian rocks. Upon 

 the whole, the Asiphonate Bivalves are more characteristically Palae- 

 ozoic, while those in which the mantle-lobes are united and there are 

 respiratory siphons, are principally found in the Secondary and Ter- 

 tiary rocks. One of the principal Palaeozoic groups is that of the 

 Aviculidce, while the Mytilidce are also largely represented. Mono- 

 myary types appear in the later portion of the Palaeozoic period, 

 numerous forms (Avicidopecten, &c.) allied to the recent Scallops 

 occurring in the Carboniferous rocks. With the commencement of 

 the Secondary period, in the Trias, many old types disappear, and 

 new ones take their places. Monomyary Bivalves are now numer- 

 ous, but among the Dimyary forms the Asiphonate families still pre- 

 dominate. The forms with long retractile siphons (Sinupallialia) 

 begin with a few types in the Trias, and gradually become more 

 numerous as we pass upwards. The Veneridce, which are perhaps 

 the most highly organised of the groups of the Lamellibranchs, ap- 

 pear for the first time in the Jurassic rocks, and increasing in the 

 Tertiaries, have culminated in the Recent period. The singular 

 group of the Rudistce is exclusively confined to the Cretaceous 

 period. 



