Herbert L. Hawkins — Studies on the Echinoidea. 437 



of structures owing to the " massive character of the latter ossicles ". 

 If the foregoing argument is sound, this difficulty disappears. Hence 

 it is with confidence, bordering on conviction, that I now express 

 the opinion that the " buccal plates " of Conulus albogalerus represent 

 the Cretaceous phase of the plating of the buccal membrane which 

 is recapitulated during ontogeny in the earliest post-larval stage of 

 Eclxinocardium flavescens. This does not necessarily imply a direct 

 phyletic sequence between Conulus and the "Heart-Urchins"; it is 

 rather a fresh illustration of morphic parallelism independent of 

 actual phylogeny. But it entirely destroys the arguments put forward 

 by me in 1911 (Geol. Mag., p. 73), and cancels the hypothetical 

 diagram there given (PL III, Fig. 8). 



3. The Perignathic Girdle. (PL XXVIII, Figs. 1-3.) 

 (a) The Angle of Inclination. (PL XXVIII, Figs. 2, 3.) 

 The elements of the girdle in the Holectypoida, when viewed from 

 within, always show a slope outwards from the peristome, thus 

 contrasting with those of the Regular Echinoids, which are com- 

 paratively vertical in direction. In Conulus albogalerus, as in 

 Discoides (see Part V of this series), the greater part of the girdle 

 reclines against, or is bevelled off from, the proximally thickened 

 interambulacra. There are thus two roughly circular rings whose 

 diameters may be measured : the actual peristome margin and the 

 upper limits of the girdle. In a young specimen of C. albogalerus, 

 (Fig. 3), where the diameter of the adoral surface is about 25 mm., 

 the former diameter is c. 4 mm. and the latter c. 65 mm. In 

 a gerontic specimen (Fig. 2), with an adoral diameter of about 48 mm., 

 the corresponding measurements are 5 mm. and 9 mm. respectively. 

 Reducing these measurements to a proportionate scale, we arrive at 

 the following result : — 



Specimens of intermediate sizes give proportionate results between 

 these two extremes. Thus the obliquity of the girdle appears to 

 increase with age. However, the vastly greater thickness of the 

 interambulacra in the old specimen automatically increases the 

 " splay" of the bevelling, and in reality the obliquity of the girdle 

 is considerably less in the gerontic than in the small specimen. The 

 approximate angles between the girdle and the plane of the adoral 

 surface in the two examples are as follows : — 



Young specimen . . . . . 12° 



Old specimen ...... 25° 



The actual inclination of the girdle in the fully grown example is 

 thus more than twice as steep as that in the young one. This 

 result has been checked by the measurement of the angles in 

 twenty-eight specimens of intermediate size. It is somewhat 

 surprising to find a decrease in obliquity during the growth of 



