196 F. A. Bather — Studies in Edrioasteroidea — //. 



formerly presented by the now vanished test as seen from the outside. 

 In this region we see the flooring-plates rising slightly above th& 

 interradial areas, then presenting a narrow flat surface, then dipping 

 down in a slightly convex curve to the middle line of the groove. 

 The groove is less deep than in the internal cast, which means that 

 the flooring-plates were thicker in the middle of the groove than 

 at its margins. Between the flooring-plates, where they bend 

 downwards, are elongate slits, wider towards the margins of the 

 groove ; these correspond to the prominences of the internal cast. 



The right anterior and left posterior grooves are still filled in 

 places by portions of matrix ; and the hollows between this matrix 

 and the internal cast enable one to reconstruct a section of the 

 groove based on actual measurement. Fig. 3 is taken from the left 



Fig. 3. — View of the left posterior groove, as preserved in the fossil, x 4 diam. 

 ext. matrix filling the exterior of the groove, int. matrix filling the interior 

 of the test. gr. groove on the surface of the latter, p. matrix filling pores 

 between the floormg-plates. 2^'- scars where the same has been broken away. 



posterior groove, at 5-5 mm. from its distal end. Here the plates 

 are about 1 mm. thick at the edge of the groove, but thin considerably 

 towards the middle line. 



A good idea of the appearance of the groove as seen from the 

 inside of the test is obtained from a wax squeeze of the internal 

 cast (PI. X, Fig. 5). 



Forbes describes the " margin of the " grooves as composed of 

 " areal or interambulacral plates bearing 2-3 short elevated transverse 

 ridges, each of which points to the origin of an ambulacral plate, 

 short and oblong ; a double series of these ambulacral plates form 

 the canal." It is hard to believe that Forbes did not understand 

 he was dealing with an internal cast ; yet this seems to have been 

 the case, for he proceeds to describe the external impressions of the 

 grooves as triangular arms " composed of two rows of dove-tailing 

 joints, with ridges at the articulations to lock into the furrows 

 bordering the arm-canal." From this truly remarkable misconception 

 arose an extraordinary theory as to the nature of the radial grooves 

 and as to the homologies of the ambulacral areas in Echinoids. 

 After half a century one need not linger over mere errors of fact 

 in the first sentence quoted. The present description and figures- 

 will enable readers to make the corrections themselves. 



Piecing all this evidence together as in Fig. 4, we see that each 

 radial groove was composed of a double row of flooring-plates, more 

 or less alternating, and meeting in a median zigzag line. In the 

 proximal half of the groove the long axis of each of these plates, 

 nearly at right angles to the middle line of the groove, measured 

 3-3-5 mm., and the short axis, parallel to the middle line, '9 mm. 

 The total number of plates along one side of a groove was about 60 



m 



