POTERIOCRINID^. — POTERIOCRINUS. 75 



objects of its sustenance were less minute in comparison with itself than its pigmy pro- 

 portions might otlierwise lead us to suppose, for it is quite certain that among animals 

 the organs for securing the food and the size and strength of tiie objects to be captured 

 invariably bear a relative proportion to each other. 



Tlie primary, and secondary rays of the Poteriocrinus isacobus, are each composed of 

 a single joint, whose length is very considerable when compared with the thickness. — 

 Inwardly they are deeply and vvidel}^ grooved, so that when the inner side is presented to 

 the eye they appear somewhat like sections of small bones. The minor rays are composed 

 of several joints, but these are much longer than usual in crinoids even of a large size. 



The final divisions of the rays, as far as we have been able to ascertain, amount to 

 twenty, and this to all appearance is the full number. 



The tentacula are few and far between. 



The beautiful little specimen from which our illustrations are taken was found in the 

 mountain limestone of Ireland, and is now in the cabinet of the Authors. 



3. Species. Poteriocrinus rostratus. (Austin.) 

 PL 9. Jig. 2 a, to2f. 



Definition. — The plates surrounding the body agree in number and arrangement 

 with those of the typical species, but they are smaller and less massive. All the periso- 

 mic plates are smooth. iMouth proboscidiform and centrical. 



Synonymes and References. 



Poteriocrinus rostratus. — Austin, Ann. Nat. Hist. Vol. 10, II. p. 108, and 196. 

 gracilis.— Griff. Carl, lime of Ireland. PL 2, Fig. 11. <o 14. 



Formation and Localities. 



Mountain limestone. — Hook Point, Wexford ; Banks of the Avon, Bristol. 

 Magnesian beds of Mountain limestone. — Clevedon, Somersetshire. 



