404 Prof. Beyricli on the Base (Pelvis) 



the case also In the diagrams which M. de Koninck has given 

 for his two species. 



Lecanocrinus is distinguished from Mespihcrinus by the 

 polar interradius, as in Poteriocrinus^ having received an 

 oblique extension or a sort of twisting by the intercalation of 

 an accessory segment sinking downwards towards the right 

 side. In consequence of this, the radial segments laterally 

 adjacent to the polar interradial space, as well as the adjoining 

 segments of the upper basal circle, have an irregularly dis- 

 torted form, which, however, exerts no influence upon the 

 division of the inferior basal circle. The division of the base 

 may be observed in the patina of an undescribed Silurian 

 species from Gotland ; it is as in Taxocrinus^ and agrees with 

 the various diagrams given for the genus — by Hall, of L. 

 macropctalus (Pal. New York, ii. pi. 45. fig. 1^), of L. ornatus 

 {I. c. pi. 44. fig. 2</), and of L. cah'adus [I. c. pi. 46. fig. 3h), 

 and by Schultze of the Rhenish L. Roemeri (Echin. des Eifler 

 Kalkes, p. 40). 



Thus, from what precedes, the observed division of the base 

 in the Taxocrinid^e was the same in all cases, which, in- 

 deed, are not very numerous ; and the agreement with the 

 diagrams cited is in favour of the condition observed being a 

 law generally prevalent for the whole family. Deviations, 

 such as are shown by Hall's diagram oi Furhesiocrinus in the 

 * Palaeontology of Iowa,' and by Schultze's diagram of Taxo- 

 crinus in the ' Echinodermen des Eifler Kalkes ' (p. 32), I 

 am inclined to ascribe to the circumstance that the authors 

 had not yet directed their attention to the relation here under 

 consideration. 



Among the Crinoids with a monocyclic quadripartite hase 

 there is scarcely one, except Melocrinusj that is fitted for corre- 

 sponding observations. As in Platycrinusj certain species of 

 this genus possess a central or subcentral vertical aperture, 

 and the five interradial spaces are alike in composition, so 

 that the calyx does not admit of being divided in accordance 

 with a radial axis. But in the majority of the sj^ecies the 

 vertical aperture has a decidedly lateral position, turned to- 

 wards one of the interradii, and the subjacent interradial space 

 acquires a greater number of plates, as, indeed, was remarked 

 by Goldfuss. In such Melocrini a definite rule for the 

 position of the dorsal axis is also demonstrable, but excep- 

 tions from this occur, which has not been observed in Platy- 

 crimis. 



The normal position of the dorsal axis in Melocrimis is dif- 

 ferent from that in the tripartite base of Platycrinus ; it runs, 



