102 GEOLOGICAL MEMOIRS. 



Devonian systems ( T, aspera and T. Wilsoni), four which are De- 

 vonian and carboniferous ( T. acuminata, T. pugnusy T. rhomhoidea 

 and T. saceulus *), two which are both carboniferous and Permian 

 ( T, Roissii and T. Schlotheimi), and two others, ( T. canalis and 

 T. elongata,) which appear to be common to the three uppermost 

 divisions of the palaeozoic period. 



The true Terebratulae are however rare in the Lower Silurian 

 rocks, and most of the species that have been quoted from such de- 

 posits must be referred either to Orthis or Spirifer. The species 

 peculiar to the Silurian system are represented in the Ural by T, 

 reticularis, T. nuda, T. camelina and T, subcamelina, and in Lithu- 

 ania and Esthonia by T. Duboisi, T. aprinis and T. Wilsoni. In 

 the Devonian system the Terebratulae become more common, and 

 one group, nowhere hitherto met with in any true Silurian deposit, 

 requires especial mention. It includes those Terebratulae covered 

 with concentric lamellar striae which never present a deltidium, 

 and of which the rounded apicial aperture is placed directly on the 

 crochet of the ventral valve. In Russia this group includes T. con- 

 centrica, T. Helmersenii, T.lamellosa, T. Roissii and T.pectinifera, 

 two of the species belonging to the Devonian system, one to the 

 carboniferous, and two to the Permian ; but it is worthy of note, 

 that T. Roissii, confined in Russia to the Permian rocks, is found in 

 Belgium to range also into the oldest carboniferous beds. Amongst 

 other Devonian species, we may mention T, Meyendorfii, widely 

 spread throughout Russia, T, Livonica, and perhaps also T, Dalei^ 

 densis, which M. Romer describes as occurring in the grauwacke 

 on the banks of the Rhine ; and we have also from the Ural T, ari- 

 maspus, T, Strajeskiana and T. suhlepida, of which the latter has 

 many relations with T. lepida of the Eifel. 



Although our own researches have proved the existence of about 

 twelve species of Terebratula in the rocks of the Russian carbonife- 

 rous system, yet these species are generally rare and limited to so 

 few spots, that the genus on the whole is by no means so abundantly 

 represented by individuals as in the districts further to the west. 

 The species T. Schlotheimi of Sterlitamak is the most remarkable 

 of the whole number, as well by its internal structure, in which it 

 resembles Pentamerus, as from the fact of its occurrence as a car- 

 boniferous fossil in Russia, although in Germany and England it is 

 confined to the zechstein and magnesian limestone. If we consider 

 the whole group of Terebratulae, we shall find that the most common 

 in Russia are T, i^eticularis, T. concentrica, T. elongata, T, Meyeri'- 

 dorjii and T.pleurodon, and all these, with one exception, are widely 

 distributed in distant countries, and extend also through many suc- 

 cessive deposits. 



In Russia we only know of one species of Strigocephalus (S. BuT' 

 tini), which occurs in limestones supposed to be of Devonian age on 

 the banks of the Serebrianka (Ural). Such is not the case with 



* We somewhat doubt whether these four species are really met with in the 

 Devonian rocks, although they are so described by Mr. Phillips (Pal. Fos. of Corn- 

 wall, &c., pp. 87,88,91). 



