194 PHOCKEDINGS OF THE MALACOLOGICAL SOCIEir. 



Monorayariiui type docs not appear till the Upper Palaeozoic forma- 

 tions are reached; forms, like Aviculopecten, allied to the recent 

 Pectens, carrying;' this type back to the Carboniferous. Four genera 

 of Lamellibranchs, namely, Pterinca. (Umucardium, Mcgahdon, and 

 Cardioniorpha, appear and disappear in Palfcozoic times from Silurian 

 to Carboniferous; -whilst -Edtiwndia, Mi/a/ina, Fosidonomya, and I'lcuro- 

 phorus appear in Palaeozoic times and extend up to the Trias. 



One genus, hoarca, appears in the l^ower Silurian, and survives to 

 the Chalk. The genus Gervillia appears in Carboniferous times, 

 and also survives to the Chalk. 



In the Trias many old types disappear, being replaced by more 

 modern forms. Monomyarian bivalves are numerous, whilst the 

 Dimyaria without siphons are still abundant. Gradually, however, 

 the forms with long retractile siphons increase, and replace the older 

 tjrpcs as we approach the newer rocks. 



Opis appears in the Trias, and ends in the Chalk ; whilst Mijoconcha 

 appeal's in the Permian, and survives to Pliocene times. 



Three genera, Grijplma, Lwceramus, and Goniomya, extend from 

 the Lias to the Chalk. 



Tancredia and Unicardium occur only from the Lias to the Oolite ; 

 whilst Exogi/ra appears in the Lower Oolite, and ends in the Chalk. 



The Venerida? appear first in the Jurassic rocks ; they increase in 

 the Tcrtianes, and culminate in our existing seas. 



One family, the Hippuritida;, including the genera Caprof/na, 

 Capn'na, Caprinella, BiradiolUes, BarreUia, Radiolites, Hippurites, 

 numbering nearly 100 species, are confined to the Cretaceous beds, 

 none being found earlier or later, unless they should be included with 

 the Chamida), which also appear in the Lower Cretaceous, but extend 

 onwards to the recent period, and are living in the seas of today. 



Of genera which have survived from very early times. Area is 

 a tinily remarkable example, being found in the Lower Silurian and 

 surviving at the present day, the living forms having a world-wide 

 distribution. The genus Pintia appears in the Devonian, and survives 

 at the present day ; whilst Lima dates from the Carboniferous, having 

 200 fossil and 46 living representatives. Cyprina appears in the 

 Muschelkalk, and survives to-day : it is represented by 90 fossil 

 species and 1 living form, C. islandica. Four genera, namely, Tri- 

 (jonia, Isocardia, PUcatula, and Cardlfa, appear in the Trias, and have 

 survived to the pi-esent day. Trigonia has more than 100 fossil 

 species and 6 recent; Isocardia has 70 fossil and 5 living species; 

 PUcatula has 40 fossil and 24 living species; Cardita has 100 fossil 

 and 72 living species. 



Three genera, Astarfe, Pholadomya, and Corhis, appear first in the 

 Lias, and have survived until to-day. Astarte has 200 fossil and 34 

 living species; Pholadomya, 150 fossil and 2 living species; Corhis, 

 80 fossil forms and 5 living species. Two genera, Limopsis and 

 Nemra, appear in the Oolite, and still survive. Two others, Unio 

 and Cyrmia, begin in Wcalden times, and still live on.^ Pectuncuhis, 



1 All these forms, save Unio aud Cyucnu, ajjpear to be uiuriue iii liabit. 



