martin: new features in uintacrinus. 195 



ing the same line. The depressions are connected with each 

 other by a narrow canal at the center of the plate it traverses. 



The radial diameter of the proximal depression exceeds the 

 transverse diameter by half its length, while exactly the op- 

 posite occurs at the point where the pinnules branch out (see 

 pi. IX, fig. 3), until at the sixth and seventh secundibrachs 

 they appear only as deep, narrow, transverse grooves which 

 extend clear across the arm plates. Each series of pinnules 

 is supplied with a set of the nerve groove depressions, which 

 branch out from the main arm grooves. 



The specimen on plate IX, figure 4, shows some well-defined 

 markings on the inner side of the plates of a portion of another 

 cup. The lines occur in three or four small terraces, which 

 conform very regularly to the shape of each plate, and must 

 have been for the attachment of several layers of ligamentary 

 muscular tissue, thus forming a much stronger, though no 

 less flexible sutural articulation than has been heretofore sup- 

 posed probably. 



Bather "^ says : "If the base be dicyclic, the ring forms a 

 commisure at the level of the centers of the basals ; and from 

 these points the cords again fork towards the adjacent infra- 

 basals, where they join in another ring round the chambered 

 organ." 



Out of the six specimens I have examined, all of which are 

 dicyclic, four show a perfect ring commisure at the level of 

 the radials ; the other tw^o show it distinctly on three of the 

 radials. I have traced a single nerve-cord groove clear from 

 the basi-infrabasal sutures to the center of the basals, at 

 which point they fork, sending out a prong towards the 

 center of each of two adjoining radials. From the above evi- 

 dences of the course of the nerve-groove system, which ap- 

 pears to be directly in opposition to the principle laid down 

 in Lancaster's Zoology, it would seem that some alteration 

 must be made in our ideas of the course of the nerve groove 

 and situation of the ring commisure in dicyclic forms, at least 

 in the case of Uintacrinus. 



A diagram based upon the material examined, showing.the 



5. Lancaster's Zoology, part HI. p. 194. 



2-Univ. Sci. Bull., Vol. IV. No. 6. 



