V. Steganoblastus. 1914 195 



Description of the Specimens. 



The measurements of the piriform theca in specimen A are 

 approximately: height, 19*5 mm. ; greatest diameter, 18 mm., this 

 being at 8*5 mm. from the summit. 



Except for the tegminals and cover-plates, all the thecal elements 

 are closely united, so that it is difficult to distinguish the sutures. 

 This difficulty is enhanced by the pitted ornament of the surface, and 

 by the very deep folding of the plates which form the adapical 

 system. On the other hand, as one removes the tenacious black 

 matrix from the depths of the folds, one realizes that they represent 

 a, system of rhomb-ridges and axial folds on the plan so common in 

 Pelmatozoa ; and, remembering that in such cases the folds are at 

 right angles to the sutures, one is able with somewhat greater 

 confidence to trace the limits of the plates. In some cases it may be 

 that the arrangement of the folds indicates rather the original con- 

 stitution of the young individual (or of its ancestor) than any actual 



Steganoblastus ottawaensis. 

 Fig. 1. Restored view of theca from left posterior interradius. 



FlG. 2. Restored view of oral surface. 

 Both drawings are based mainly on specimen A. x 2 diam. 



separation of plates in the adult. It seems plain that the tendency 

 of the species is to fuse the originally indefinite smaller plates into 

 a few larger plates of definite arrangement (cf. Text-fig. 3). 



The plates distinguishable in the adapical system (Text-fig. 1) are 

 five basals, five radials, and in each interradius a varying number of 

 interambulacrals. From a strictly morphological standpoint, one 

 should perhaps also include in this system the floor-plates of the 

 subvective grooves. 



The Basals are of the pentagonal shape usual in normal crinoids. 

 As the following measurements from specimen A show, they are 

 a little lower on the posterior side of the theca, owing to the space 

 required by the anal plates. The measurements are in millimetres 

 and are taken along the surface of the theca. 



l.ant. l.post. post. r. post. r. ant. ant. 

 Height of basal . . . . 5'6 5*5 51 5'2 5 7 



Height of interbasal suture 4*6 4*0 3*5 4'2 4*25 



