PAL^EOCRINOIDEA. 



427 



Among the other genera of the Acti?iocri?iida? may be mentioned 

 Stelidiocri?ius, Briarocrinus, and Carftocrinus, which have been regarded 

 as the types of as many families, and all of which are found in the 

 Silurian rocks. 



Family 2. Barrandeocrinidce. — This family has been founded by 

 Angelin for the reception of the single genus Barra?ideocrinus, which 

 occurs in the Silurian rocks of Gotland. In this genus the arms 

 are arranged in pairs, and are reflexed in such a way that their 

 dorsal surfaces rest upon the outer surface of the calyx, while they 

 are at the same time confluent laterally. 



Family 3. Platycrinidce. — This family is characterised by the 

 possession of a " monocyclic " calyx, with three, or sometimes two, 





Fig. 300. — Platycrinus tricontadactylus. Carboniferous. The left-hand figure shows the 

 calyx, arms, and upper part of the stem, and the figure next this shows the surface of one of the 

 joints of the column. The right-hand figure shows the proboscis. 



basals and five radials. There are from three to five plates in each 

 interradial space ; but the anal interradials do not come in contact 

 with the basals, except in some forms (Hexacrinus). The arms are 

 at least ten in number (fig. 300), and may be uniserial or biserial. 

 The column is round or elliptical, and the neurovascular axial canal 

 is of small size. The geological range of the family is from the Silu- 

 rian to the Carboniferous inclusive, but the maximum development 

 is attained in the Carboniferous Limestone. 



The type-genus of the Platycrinidce is Platycrinus itself, in which the 

 cup is composed of three basals and a single cycle of radials, amongst 



