96 POTEEIOCRINIDvE.— EXTRACEINUS. 



Miller derived the generic name Pentacrinus from the pentagonal shape^of the column, 

 a form of structure not peculiar to the crinoids of the lias, or more recent formations, 

 though generally very characteristic of them. There is one character however which 

 distinguishes the pentagonal columns found in the lias and more modern strata from 

 the pentagonal columns of the crinoids which existed in the carboniferous seas, namely, 

 the former invariably articulate by pentagonal crenated star-like forms, while the 

 articulating surfaces of the latter are furrowed by linear striae radiating from a central 

 axis. 



So little inclined have we been to multiply genera or species unless when circum- 

 stances imperatively demanded it, that we have already published in our Monograph a 

 plate with a beautiful group of crinoids under the title of Pentacrinus Briareus, and 

 which at the time of the publication we hojjed might still be considered as appertaining 

 to that genus, but on comparing these fossil species with the recent Pentacrinus it was 

 found incompatible with any kind of systematic arrangement to continue in one and 

 the same genus animals differing so materially from each other as the Pentacrinus Caput 

 Medusa, and the Extracrinus liriareus. In the latter the body is of considerable size, 

 enlarging within the rays into a capacious abdominal pouch, but in the true 1 eutacrinus 

 it is extremely diminutive, and the second series of perisomic pieces are never jointed, 

 or much produced beyond the margin of the dorso-central plates as they are in the 

 Extracrinus, which has in consequence of these additional ossicula several more jjieces 

 in the frame work of the body than the Pentacrinus Caput Medusce. 



1. Species. Extracrinus Briareus. {Miller spec.) 

 PL 12, Fig. 1 rt, to r. — also the first plate. 



Definition. — Dorso-central plate composed of five pieces ; large series of lateral pieces 

 apparently five, but as each of these is jointed they amount to fifteen or twenty accord- 

 ing to age, with five small wedge-shaped pieces inserted alternately with the larger 

 ones, which latter are prolonged below the margins of the dorso-central plate ; mouth 

 centrical and slightly protrusive, but not proboscidiform ; rays numerous and closely 

 set with tentacula ; column long, and furnished with numerous auxiliary side arms. 



