PAL.EOCRINOIDEA. 



43 



centre of the vault 



occupied by five oral plates, while the 

 and their lateral branches are 

 covered in by small calcareous plates. The genera of this family are 

 Silurian, but Ckiocrinus, if rightly referred here, is Ordovician. 



ambulacral grooves of the arms 



The type-genus of this family is Crotaloc?'inus (A?ithocrinus), one of 

 the most beautiful Crinoids of the Wenlock Limestone of Gotland and 

 Britain. The calyx consists of five small underbasals and five large 

 basals, with a single zone of radials, while interradials are wanting, 

 with the exception of a single small anal plate. The arms are bifur- 

 cated, and the subdivisions unite with one another by means of lateral 

 processes, thus giving rise to a network, perforated by numerous aper- 

 tures (fig. 303). The Silurian genus Enalloc7'inus is nearly allied to 

 Crotalocri?ius, but the arms become free towards their extremities. 



Family 7. Ichthyocritiidce. — In this family the calyx is " dicyclic, : 

 but the three underbasals are 

 small, and are mostly not visible 

 externally. The radials articu- 

 late upon one another, and are 

 united laterally by the peri- 

 somic plates of the disc. The 

 arms are short, bifurcating, and 

 often in contact laterally, thus 

 forming an upward continua- 

 tion of the calyx (fig. 304, a) ; 

 while pinnules are apparently 



wanting. The disc had 



open 



ambulacral grooves, and was 

 covered with perisomic plates, 

 like that of a recent Crinoid. 

 Its central part was occupied 

 by five oral plates, which in 

 some species, if not in all, 

 were separate from one another, 

 so as to open the mouth to the 

 exterior. The members of this 

 family are found in the Silurian 

 deposits. 



The typical genera of the IchthyocrinidcB — such as Ichthyocrimts itself 

 (Silurian to Carboniferous), Lecanocrinus (Silurian and Devonian), and 

 Mespilocrinns (Carboniferous) — are characterised generally by the im- 

 perfect separation of the calyx and arms, the latter being commonly 

 in close apposition laterally (fig. 304, a). On the other hand, the genus 

 Taxocrinus (fig. 304, b) represents a section of the family — sometimes 

 raised to the rank of a separate family ( Taxocrinida?)—m which the 

 arms are well developed and repeatedly bifurcated. The species of 

 Taxocrinus are found in the Silurian, Devonian, and Carboniferous 



Fig. 304.— a, Side-view of the calyx of Ichthyo- 

 crinns Icevis, from the Silurian (Niagara Lime- 

 stone) of North America ; k, Calyx and upper 

 part of the column of Taxocrinus tuberculatum, 

 from the Silurian (Wenlock Limestone) of Brit- 

 ain. (After Hall and M'Coy.) 



Devonian, and Carboniferous 



