468 MR. HERBERT L. HAWKINS ON 



D. The A m b 11 1 a c V a. 



1. The Poclial Pores. 



Although one of the diagnostic characters of the Holectypoida 

 consists in the aptjtaloid nature of the ambulacra, it would be a 

 mistake to assume that the pore-pairs are therefore similar 

 throughout the group. The Nucleolitidse, which are a group of 

 almost the same antiquity as the Holectypoida, early developed 

 a marked heteromorphy in the podial pores of the adapical 

 surface ; and a similar character, continually recurring, but as 

 often held in check, is apparent among all the Jurassic members 

 of the order. Pygaster sens, lat., has uniformly larger pores on 

 tlie adapical than on the adoral sui'face, and the members of an 

 individual pore-pair are dissimilar in the former region of the test. 

 Even Pygaster semisulcatus sometimes shows this feature. The 

 outer pore of the pair is a little larger than the inner, although 

 both are somewhat elliptical in shape. In P. {Megapygi(,s) umbrella, 

 and still more in M. inacrocyphu.s, the diiference becomes increased. 

 The inner pore is cii"cular, and the outer retains an elliptical 

 shape, often on quite an elongated plan. The long diameter of 

 the outer, elliptical pore never becomes more than twice as great 

 as the diameter of the inner, circular one ; so that . the whole 

 ambulacrum cannot be said to show even a subpetaloid structure. 

 In Pygaster, while this dimorphism of the adapical pore-pairs 

 increases, the size of the pores on the adoral surface steadily 

 decreases. These latter pores are always circular, and the members 

 of each pore-pair are sepai-ated by a prominent granule. Their 

 small size renders them quite difficult to distinguish in the 

 Upper Jurassic forms. 



In Holectypus a similar tendency is seen, although it is hardly 

 appreciable in the Cretaceous subgenus. The diversity of shape 

 and size in the adapical pore-pairs is rarely carried so far as in 

 Pygaster, but the reduction in the diameter of the ambital and 

 adoral pores is quite as well marked. In the case of Discoidea 

 the tendency is less noticeable. The pores of the adapical 

 sui-face are only very slightly larger than those of the adoral 

 (both series being minute), and are themselves always circular'. 

 The outer member of a pore-pair is sometimes just distinguishable 

 from the inner One in point of size. 



In Conulus, by way of contrast, the pores of the ambulacra are 

 everywhere exceedingly minute, those of the adapical surface 

 being even smaller than those of the adoral. The largest pores 

 in this genus are generally situated on or near the ambitus. 



Discoidea, and to a further extent Cooiuhis, may be regaided as 

 illustrating the triumph of simplicity of ambulacral strnctiu'e over 

 the persistent tendency to complexity which induced variation in 

 the earlier genera. In the case of Discoidea, the simplicity would 

 seem to have l)een short-lived as soon as its successor, Conoclypeus, 

 had emerged from the order Holectypoida, and initiated the 

 Clypeastroida, where often the petals are developed to a great 



