486 MB,. HERBERT L. HAWKINS ON 



stock, for the anomalous biserial ambulacra are unlike any other 

 genera of the order. The periproct position shows an advance on 

 the 3Iega/pygus-condition, and, owing to certain irregularities of 

 the tuberculation, I am inclined to regard Piletts as a side-branch 

 of that line. Anorthopygus, which in the classification I have 

 associated with Fileus, seems to show a course of evolution 

 parallel with, althovigh in many ways differing from, that of the 

 aberrant genus. The oblique position of the periproct does not 

 appear to be an important character, although pecviliar. The 

 tuberculation is definitely like that of Macropygus in structure, 

 though not in arrangment, and therefore I have regarded it as an 

 offshoot from that svibgenus in Lower Cretaceous times, which 

 corresponded with the similar offshoot from the Megapygus-line 

 in the Upper Jui'assic. 



At about the same horizon in which Anorthopygus occurs, 

 Conulus appears. The earlier species seem very difficult to dis- 

 tinguish from those of Pyrina with which they may be strati- 

 graphically associated. In the matter of the tuberculation the 

 adoral surface of Conulus shows much the same characters as the 

 whole test of Anorthopygics. Moreover, the arrangement of the 

 tubercles is similar in both genera. The periproct has passed to 

 the posterior edge of the test, although in many young specimens 

 of G. suhrotund'us (some of which are almost globular), the aper- 

 ture is on the adoral surface quite near to the apex. The feature 

 which marks oflf Conulus so sharply from the Pygasteridje is the 

 accelerated condition of the ambulacral plate- crushing. There 

 is no appreciable tendency to increase the number of demi-plates 

 in the Pygasteridfe, from the few adorally situated ones, which 

 were probably directly inherited from the Diademoid ancestor. 

 However, in many other features Conulus shows almost equal 

 acceleration. "When the Upper Chalk is reached, the genus 

 disappears suddenly after a short existence, during which few 

 important specific modifications were evolved. Its relations to 

 the Pyga^teridfB are not very easy to decide, but, on the character 

 of the tuberculation, I have connected it with the Anorthopygus- 

 line. An additional link between the genera is afibrded by the 

 structure of the apical system, the fifth genital plate being per- 

 manently absent from both. 



4. The DiscoidiidiJe. 



Holectypus sens. str. appears in the Inferior Oolite in asso- 

 ciation with Pygaster sens. str. It is only in the position of the 

 periproct that considerable acceleiation is shown, but the dif- 

 ferentiation of the characters of the tubercles on the upper and 

 lower surfaces of the test is also a feature of advance. The 

 Holectypinse are a perfectly homogeneous group, and must be 

 regarded as an unbroken series. Coptodiscits is apparently a 

 peculiarly specialized offshoot from Co&nliolectypus^ and the 

 suturing of the adapical surface may perhaps be ascribed to 



