100 GEOLOGICAL MEMOIRS. 



rossicuSf so widely spread in the carboniferous system of that coun- 

 try, really belongs to the genus Archceo-cidaris, M^^Coy (JEchino- 

 crinus, Ag.), which indeed appears to be limited to this part of the 

 palaeozoic series in Europe. 



The Crinoidea, generally so rich in varieties of form in the pa- 

 laeozoic rocks, are limited in Russia, so far as regards the three 

 upper portions of the series, to detached fragments of the stem or 

 arms, of which the species is rarely determinable. The only perfect 

 head that we have met with is a specimen of Cupressocrinites nud- 

 formis^ Fisch., from the carboniferous limestone of Miatchkova. 



In the Lower Silurian rocks of St. Petersburg, and on the shores 

 of the Baltic, we observe a curious relation among the specimens, 

 the instances of the perfect heads being more frequent than the 

 occurrence of the fragments called Entrochi ; but this is readily ex- 

 plained by the fact, that most of the Crinoids of this epoch belong 

 to genera deprived of arms, and having only a short and rudi- 

 mentary stem. It is to these genera that M. von Buch has given 

 the name Ci/stidea'^, grouping them into a family of which the 

 principal distinguishing characters are the absence of true armsf 

 and the presence of an ovarial orifice on the side of the cup. This 

 family is the more interesting to the palaeontologist since it seems 

 to have preceded the other Crinoidea in order of time, and presents, 

 as it were, the primitive form of animals of this class, since most of 

 the genera of which it is composed are peculiar to the lower por- 

 tion of the Silurian system, and disappear entirely where that ter- 

 minates. In Russia the family is represented by the geuerai Echino- 

 sphcBriies(^SpkcBronites),Ec/d?io-encrimtes, Hemicosmites and Crypto- 

 crinites. In Sweden it is limited to the first two, in England to the 

 first only |, while in the rest of Europe, and even in America, it is 

 doubtful whether there is a single representative. EchinosphcBriies 

 aurantium is one of the most common of the Lower Silurian fossils 

 both in Russia and in Scandinavia. E. pomum is, however, much 

 more rare, and E. balticus seems peculiar to Russia. The species 

 Eo tessellatus, of which the true nature is very imperfectly known, 

 is found higher in the series, and both in the Ural and in Devon- 

 shire occurs in Devonian rocks. 



In glancing over the general list of palaeozoic fossil Crinoids, we 

 find the genera Actinocrinites, Asterocrinus, Cyaihocrinites, Euge- 



* See Quart. Geol. Journ. vol. ii. Part 2. p. 20. 



t The very delicate tentacula which M. Volborth has recently discovered in the 

 genus Echino-mcrinites (Bull, de la CI. Phys. Math. St. Petersb., vol. iii. No. 6) 

 are placed on the very edge of the orifice of the mouth, and do not penetrate the 

 plates as the arms of ordinary Crinoidea do. It also appears that these were not 

 provided with a receptacle intended to contain eggs, since the genus Echino-encri- 

 nites {Sycocystites, v. Buch) has, according to M. von Buch, a large ovarial orifice. 

 [The authors figure a specimen of Cryptocrinites, and state that on this speci- 

 men five small depressions may be distinguished round the mouth which appear 

 to have been points of attachment of similar tentacula. — Ed.] 



% Echino-sphcerites aurantium has lately been discovered by the officers of the 

 Geological Survey in Pembrokeshire, in those rocks which have been described by 

 one of the authors as Lower Silurian. 



