POTERIOCEmiDiE.— POTERIOCRINUS. 91 



In general outward appearance, some species of Poteriociinus bear a slight resemblance 

 to animals of the genus Cyathocrinus, but when the calcareous skeleton is examined with 

 proper attention, differences sufficiently obvious, will present themselves to notice, and 

 prevent all chance of confusing the animals of one genus, with those of the other. 



The group of inter-radial plates at one point of the circumference, and which appear 

 designed to connect the proboscidiform oral tube more completely with the body, than if 

 the rays were equally close all round, and the base of the proboscis had only the inner 

 edges of the ray-bearing plates to rest on, is alone sufficiently characteristic of the Poterio- 

 crini, as a single hexagonal inter-radial plate, resting on the upper edge of the hexagonal 

 ' piece of the first series of perisomic plates, and forming the interspace, is a distinctive 

 mark of the Cyathocrini, which hav'e mouths protrusive but not proboscidiform. The 

 difference in the dorso-central plates, and the manner in which the rays articulate as 

 previously noticed, are also distinguishing characters, which cannot easily be mistaken. 



The irregularities before adverted to, as occurring in some of the older specimens of 

 Poteriocrini, more particularly in Mr. Morgan's specimen. Pi. 8. Fig. 3a, and b, are clearly 

 the result of injuries, not of original structure. The irregularities in the rays, and other 

 malformations are mostly found in specimens which exhibit signs of age,and which so often 

 appear to have suffered injuries of various kinds ; but the younger animals are mostly free 

 from deformity, and possess the same symmetrical forms, as those of other genera. 



In some species of Poteriocrinus we can trace characters which seem to connect them 

 remotely with several other genera. The P. granulosus in the thickness of its lateral plates, 

 and contracted cavity for the digestive organs, shews its distant affinity to the genus 

 Apiocrinus ; and the P. Latifrons when the rays are closed around the oral tube suggests 

 a comparison with the Encr'mus monilij'ormis. 



It should have been observed that in noticing the different columns, the airangement 

 of the ossicula as seen in the most perfect specimens, has been the chief evidence on which 

 our descriptions are founded, but at the same time we are aware that the very thin alternate 

 joints near the summit are frequently the new and imperfect joints, interposed between 

 the older and more perfect articulations. 



When the number of species is considered, as well as the relation they bear to other 

 groups, together with their variations in form and structure, the Poleriocrinus becomes 

 one of the most important genera, among the numerous and varied forms of the 

 Crinoidea. 



The gradual development of new species, exhibits in an interesting manner the 

 constant accession to our knowledge of these invertebrate animals, which formed so 

 important a part among organic beings ere the higher organized animals held undisputed 

 dominion over land and sea. Mr. Miller in his work on the Crinoidea, describes but 



