106 GEOLOGICAL MEMOIRS. 



cies sometimes induces uncertainty in the determination of them, 

 a still greater difficulty, and one of a different kind, is presented, 

 when we consider the remains of Acephala in the palaeozoic rocks ; 

 for most of the specimens having their valves either closed or buried 

 in the rock, it is often impossible to recognize those characters on 

 which genera are usually based, such as, for instance, the arrange- 

 ment of the various parts of the hinge, the number of the teeth, the 

 form of the muscular and palleal impression, and the position of the 

 ligament. It will readily be seen how much this uncertainty di- 

 minishes the value of these fossils, and the limited use we are able 

 to make of them in their application to geology. 



The Tubicolaria, Pholadaria and Petricolaria are certainly absent 

 in Russia, as elsewhere, in the palaeozoic rocks. The Solenacea 

 are only represented by the genus Solemya, of which there are 

 two species, one of them Permian and the other carboniferous ; and 

 the new genus Allorisma, of the same family as Pholadomya, also 

 presents two species, one of them, A, regularise resembling A. sul- 

 cata (^Sanguinolaria, Phillips), very common in the carboniferous 

 system both in Russia and England. We may mention here also a 

 small shell, which we have described as an Osieodesma, though not 

 without special notice of the singularity of a genus M^hose develop- 

 ment is recent commencing so low in the series of beds. The two 

 genera Amphidesma and Edmondia are the only representatives of 

 the family Mactracea, which was afterwards to take so much more 

 important a position among marine animals at the sea-bottom. Ed- 

 mondia unioniformis, which lived during the carboniferous epoch, is 

 common to England and Russia. The Cardiacece, now met with in 

 the sea in all latitudes, commenced their existence at the earliest pe- 

 riod, but the number of species in the older rocks is far less consider- 

 able in Russia than in the rest of Europe ; and the genus Cardium, 

 which alone includes fifty palaeozoic species, is there represented by 

 only five or six, of which three belong to the division Pleurorhynchus. 

 These species all belong to the carboniferous system, with the ex- 

 ception of C. striatum^ which M. Pander states he has found in the 

 island of CEsel, and C, palmatum, which we have observed among 

 Nova Zembla fossils. We also refer to this latter species a small 

 shell found in the bituminous schists (Domanik-schists) on the 

 banks of the river Uchta, associated with Goniatites resembling 

 those which in Westphalia accompany the very same species. The 

 genus Cardiola, which is tolerably abundant in England and Nor- 

 mandy, and extends as far as the Fichtelgebirge and the environs of 

 Prague, does not penetrate into Russia; but this is not the case with 

 the Cypricardia, of which there are two species, one of which, C 

 rhombea, presents no modification of form, whether exhibited in the 

 Silurian rocks of the Ural, in Belgium or in England. 



Among the species of JVueida and Area which are described, and 

 of which there are three of the former and four of the latter, we 

 may mention N. Kazanensis as peculiar to the Permian beds, and 

 A. Oreliana to the Devonian series. In the latter the serial teeth are 

 almost entirely effaced. The Nuculas and Arcs formed with the Car- 



