GEOLOGICAL DISTRIBUTION OF LAMELLIBRANCHIATA. 271 



series from the Cretaceous, no less than two hundred and fifty out 

 of a round six iiundred from the Silurian deposits are stated to occur 

 in Northern and Western Europe. Some seventeen or more species 

 out of the fifty-two recorded by Etheridge from the middle and 

 upper Paleozoic divisions of Australia find their analogues in the 

 equivalent deposits of Europe, and not unlikely this identity will 

 be increased on further comparisons being made. The Tertiary 

 species of the two regions, on the other hand, are almost without 

 exception distinct, and of the recent forms it may be doubted 

 whether there is a single species held in common. 



The geological distribution of the Lamellibranchiata may be 

 considered to run parallel with that of the Gasteropoda, and in a 

 general way to partake of its peculiarities. Most regions are en- 

 tirely deficient in Cambrian forms, and even in the Lower Silurian 

 formations the number of species is rather limited. Barrande enu- 

 merates upwards of eleven hundred species from the Upper Silurian 

 formation of Bohemia alone, nearly double the number (636) that 

 was assigned by Bigsby in 1868 for the Silurian deposits of the 

 world generally, and considerably over that (918) which was claimed 

 by Bronn in 1863 for the entire Paleozoic series. The oldest forms 

 — i. e., Silurian and Devonian — belong almost exclusively to the 

 Heteromyaria (Aviculidae, Mytilidos) and the Dimyaria (Nuculidae, 

 Arcadffi, Astartidfe, Cardiida3), although many of the Devonian 

 forms that have been referred to the Heteromyaria may really be- 

 long to the Monomyaria. The Sinupalliata among the Siphonida 

 appear to be completely wanting, as they are likewise from the 

 Carboniferous deposits, but it is by no means unlikely that some 

 of the commoner genera, as Grammysia, Allorisma, Sanguinolites, 

 Edmondia, &c., in which no sinual impression has been detected, 

 are trlie members of families whose modern representatives are all 

 furnished with retractile siphons. This supposition, which is based 

 upon external resemblances and habits as deduced from the shell, 

 is in full consonance with the theoiy of evolution, which would 

 lead us to suppose that the direct ancestors of the Sinupalliata were 

 closely resembling forms devoid of a sinual inflection. The refer- 

 ence of the forms above mentioned to the family Pholadomyidse 

 may, however, still be considered uncertain. 



The Carboniferous Lamellibranchiata do not differ very broadly 

 from those of the j)receding (Devonian) period, except in so far as 



