LECANOCRINIDAE 1 23 



This family embraces an assemblage of genera most strongly contrasted 

 with the Ichthyocrinidae in the condition of the infrabasals, which are erect, 

 helping to enclose the visceral cavity. This is typified in the genus Lecanocrinus, 

 where they form an upright cup resting upon the column. In two of the genera, 

 Homalocrinus and Calpio.crinus, the prominence of the infrabasals is exagger- 

 ated to a high degree of specialization, overshadowing all the other basal struc- 

 tures. This modification was short-lived, and was not repeated elsewhere 

 among the Paleozoic crinoids. Correlated with the above-mentioned infra- 

 basal character in the family is the small, short and subglobose habitus of the 

 crown. The arms are usually short, or appear so by being closely folded over 

 the tegmen, producing a more or less compact, ovoid body. This character is 

 quite constant throughout the genera, Cholocrinus alone showing a marked 

 departure from it. Asaphocrinus has a somewhat less globose form than most 

 of the others, and also exhibits a tendency toward the anal structure of the 

 Taxocrinidae. The only genera of the other families which are liable to be con- 

 founded with those of this family by reason of similarity in general form and 

 habitus, and the infolding of the arms, are usually larger and are readily dis- 

 tinguished by their other characters. Various stages of anal structure are 

 progressively exhibited in this family, from those of radianal in primitive 

 position, oblique, or anal x without radianal in the Silurian, to complete elimi- 

 nation of both radianal and anal plates in the Carboniferous. 



The family is essentially Silurian, eight of its ten genera belonging to that 

 age, only two of which, Lecanocrinus and Pycnosaccus, are feebly represented 

 in the Devonian. The two Carboniferous genera, Mespilocrinus and Niptero- 

 crinus, which are morphologically the direct successors respectively of Lecano- 

 crinus and Pycnosaccus, are confined to the lower part of the Lower 

 Carboniferous, and the family, so far as known, became extinct at the close of 

 the Burlington limestone. Five of the genera, Lecanocrinus, Mespilocrinus, 

 Anisocrinns, Hormocrinus and Pycnosaccus, are common to both Europe and 

 America. Homalocrinus and Calpiocrinus are confined to England and Got- 

 land, while Asaphocrinus and Nipterocrinus have not been found outside of 

 America. 



