REVIEW OF THE PERMIAN FOSSILS. 
209 
their existence into the Permian aera, whilst twenty new species complete the total 
number which researches have shown to have occurred in this last period. 
Passing now to the Conchifers of the order Dimyaria, we may state, that whilst 
upwards of 120 species have been detected in the Carboniferous strata, their num- 
ber is reduced to twenty-six in the Permian system. Among the most characteristic 
of this family, the genus Modiola is extensively found through Russia and England. 
In the former country, our Modiola Pallasi is as certain a type of the age of the 
rocks in which it occurs, as the Productus Cancrini (nob.) : in England the charac- 
teristic Modiola is the M. costata. The genus Axinus 1 , so very abundant in the 
Magnesian Limestone, and so peculiar to that rock, has its Russian representatives 
in the A. Rossicus (nob.), and in the A. pusillus. 
Among the Monomyaria the genus Avicula is nearly as important as the Mo- 
diola in the Dimyaria. It contains eight species, all of small size and generally 
smooth. The best known in Western Europe are the Avicula Iceratophaga, A. 
antigua, and A. speluncaria. One of the valves of the last mentioned has a gryphoid 
form, and bears a great resemblance to our Russian type, A. ICazanensis. 
The number of the Monomyaria, amounting to about seventy-five in the Car- 
boniferous epoch, is reduced to sixteen in our system, and fifteen of these are 
peculiar to it. The Avicula antiqua, found by ourselves in the carboniferous 
limestone of Yitegra and of Malay oraslovetz, between Kaluga and Moscow, is 
the only species of the genus which is common to this and another palaeozoic 
zone. 
The Gastropods appear to have undergone much diminution in the formation 
of the Permian strata, and to have had great difficulty in accommodating them- 
selves to new conditions. For, if we pass over the seven minute species of Turbo 
and Rissoa, occurring in one locality only near Manchester 2 , the number of Gastro- 
pods known throughout England, Germany and Russia, in rocks of this age, amounts 
but to fifteen species, a number wdnch must appear still more insignificant, when 
we reflect, that as many as 225 species of this class are known in the Carboniferous 
system. These fifteen Permian species are almost all new ; three only having been 
able to live on from the Carboniferous to the Permian epoch. The rarity of indi- 
vidual Gastropods which are met with in the strata, seems to combine with the 
' Schizodus, MSS., King. See Observations, Table, p. 224. 
a This deposit is described by Mr. Binney, 1st vol. Trans. Geol. Soc. Manchester, and the shells are 
determined by Mr. Brown. 
