456 MR. HERBERT L. HAWKINS ON 



periproct is often situated on a considerable projection of the 

 posterior interradius. The renewed appearance of bilateral sym- 

 metry under these conditions seems to confirm the belief that, so 

 far as the Holectypoida are concerned, deviations from radial 

 symmetry are connected with the migrations of the periproct, and 

 depend upon them, having i;io real significance of their own. 



2. The Adapical S'urface. 



With the exception of one rare form {P. semisulcaUiS var. 

 conoideus), all the species of Fygaster are depressed. The conical 

 shape is not quite regular owing to the interference of the peri- 

 proct, and the apex is usually a little to the rear of the centre. 

 In Holectyptis the cone is rarely less elevated than in an aA^erage 

 Pygaster, and is usually considerably higher. The extreme flat- 

 ness of Anorthopygus orbicularis is qviite exceptional for the 

 group. The cylindrical form of Discoidea cylindrica is all the 

 more curious because of the normally conical shape of the other 

 and smaller species of the genus. The outline of Comdzis suh- 

 rotundus sometimes resembles it, but the A^ertical character of the 

 sides is never so complete. The Upper Chalk Comdi tend, as a 

 rule, to assume an acutely conical shape, so that the area of the 

 a,dapical is sometimes twice as great as that of the adoral surface. 

 The apex of the test is practically central in all the genera except 

 Pygaster. 



Among outside genera, almost the only forms to show the 

 sharply conical shape of the Holectypoida are Conoclypeus and its 

 allies. Most of the Clypeastroids are exceedingly flat in shape, 

 while the bilaterally symmetrical groups natiu'ally cannot be 

 compared with the Holectypoida in this feature. 



3. The Adored Stirface. 



There is a veiy constant progressive change in the form of this 

 region of the test within the group. From Pygaster, with a base 

 so concave that specimens placed with the mouth downwards 

 rest on the ambitiis alone, to Comdus, where the base is to a con- 

 siderable extent convex, CA^ery gradation may be traced. As this 

 feature is directly associated with some of the peristomial char- 

 acters, it will be better to postjjone its discussion to that section 

 of the paper. 



B. The Peristome and Associated Structures. 

 1, The Peristome, 



The central position of the peristome is constant throughout 

 the group. In its size there is a progressive reduction traceable 

 through the Jurassic to the Cretaceous forms. In Pygaster the 

 peristome is of about the same size, relatively to the test diameter, 

 as in an average Diademoid (about one fifth), In Holectyptis a 



