STRUCTURE OF THE CYSTIDE^. IT 



length upon the surface of the body. In Amygdalocystites florealis 

 and the two species 'of Malocystites, the arms are also recumbent, 

 but their position is somewhat different. The grooves are not in 

 the centre of the upper surface of the arras, but upon one side, and 

 there is but one row of pinnulse. These characters are not the 

 results of a different structure, but are occasioned by the curious 

 position of the arms, which do not rest with their backs in contact 

 with the surface, but with one of their sides undermost. 



3. CystidecB with free arms. — The only species known is Comaro- 

 cystites 'punctatus (Billings), which has not only the free arms but also 

 the pinnulae of a true Crinoid. It is probable that some of the small 

 Cystideans described by Professor Forbes, in the " Memoirs of the 

 Geological Survey of Grreat Britain," belong to this group. . The four 

 little prominences on the top of Caryocystites munitus appear to be the 

 remains of arms which were free, and of a large size in proportion 

 to the magnitude of the body. The genus Pleurocystites has two 

 appendages which are more of the nature of pinnulae than arms. 

 They are composed of a double series of joints, and have the grooves 

 bordered by small marginal plates. In this respect they exactly 

 resemble the pinnulee oi Pentacrinus caput- ^ledusce as figured by Miller. 

 Although in the descriptions of the species I have called them arms, 

 I am not at all satisfied that they are entitled to be so designated. 



The distribution of the arms of the Cystideas above given into 

 three kinds, is not intended as a classification of the species into 

 groups. On the contrary, we find that widely different genera, such 

 as Malocystites and Apiocysf.ites, have recumbent arms, and others 

 equally far apart, such as Echino-encrinites and Glyptospharites, with 

 pinnulae only, while Comarocystites, which agrees with the Sphceronites 

 in the numerous plates ' of the body, has the arms free. Even in 

 the same genus we have two of those degrees of development, for 

 Glyptocystites Logani has only pinnulse, but G. multipo7'us has both 

 recumbent arms and pinnulae. It is quite clear therefore that such 

 characters are not often of more than mere specific importance in 

 classification. 



V. The Calycine Pores or Pectinated Rhombs. 



Many of the Cystidese were also provided with a peculiar system, 

 consisting of pores which penetrated through the plates of the body, 

 and probably served as media of communication between the interior 

 and exterior, although the precise nature of their functions has not 



B 



