REPORT ON THE CRIN'OIDEA. 271 



this type really are different from those of other Pentacrinidae, has re-established the genus 

 Balanocrinus upon them. The five sectors of each more or less circular face have no 

 ridses or denticulations along their sides, those beinor limited to the outer marmn of the 

 joint-face. They are usually therefore of greater size than the corresponding parts on 

 the stem-joints of Extracrinus and Pentacrinus, which are sometimes much constricted 

 by the development of ridges at their sides. The stem-joints of Balanocrinus, therefore, 

 are somewhat like those of MUlericrinus ; though in the latter type the whole joint- 

 face is uniform in character, and not marked out into sectors as is the case in the Penta- 

 crinidae. Many PaUeocrinoids have joints somewhat like those of Balanocrinus, i.e., 

 crenulated round the edge, but nothing more. The genus commenced with Pentacrinus 

 in the Trias, and survived to the Lower Xeocomian, no remains of it having vet been found 

 at any higher horizon ; while I have not met with this simple form of stem-joint in any 

 recent species. 



Owing to the deficiency of our knowledge respecting the nature of the calj-x of 

 Balanocrinus, I have found it necessary to use the characters of the stem as the basis of 

 the classification of the family. Balanocrinus may have under-basals Kke Extracrinus, 

 or more than three radials like Metacrinus ; but until we know more about its calyx a 

 classification of the Pentacrinidae must depend primarily upon the varying features of the 

 stem. 



I. Five to eight large teeth at the eides of each petaloid sector, most of which start from the outer edge 

 of the joint-face, while the remainder meet their fellows in the interpalmar spaces. 



1. Three radials, ......... Pentacrinus. 



2. More than three radials, ....... Meiaerinug. 



IL Sectors linear with delicately crenulated edges. Under-basals. The first radials much prolonged 



downwards. Secondary arm-trunks each bear a succession of armlets on the same side, Extraeriniu. 

 IIL Joint-faces crenulated round the edge only, not along the sides of the sectors, . Balanocrinus. 



Three other supposed genera have also been referred to this family. One is Isocrinus, 

 von Meyer,^ of which the stem is scarcelv known : while it is probable that von Meyer's 

 description of the primary rays as consisting of but two joints, basals being likewise 

 wanting, is also somewhat incorrect. Tlie mode of division of the rays, on which von 

 Meyer laid considerable stress, is perfectly normal. I prefer therefore to refer the tjrpe, 

 temporarily at any rate, to the genus PerUacrinus, as has been done by Bronn and 

 others, though I will not attempt to foUow them into specific details. 



Another unrecofirnised crenus of the Pentacrinidae is the Chladocrinus of L. Agassiz.* 

 After defining the stem of Pentacrinus as "portant de distance en distance des rayons 

 simples verticilles," he continued, "on pourra designer sous le nom de Chladocrinus les 

 especes dont les rayons accessoires forment des verticilles plus ou moins distans." 



' Jsoerintis und Chehcrinus, Mnseiun Senctenhergianum, Frankfort, 1837, p. 251. 



* Prodrome d'une Monographie des Baxliaiies ou Echinodermes, Mem. de la Soc des Sci. Xai. dc Xeuchaifl, t. i., 

 1835, p. 194. 



