﻿Vol. 66.~\ DERBYSHIRE AND NOXTINGH AM SHIRE COALFIELD. 34$ 



rims of the opercular flap. In their specialization these rims have sometimes 

 protruding serrulas, which interlock and form a close-set grating, which 

 admits water for the respiration of the embryo, and which later breaks open 

 to permit the young fish to escape from the capsule Ventilating apertures- 

 are also present at the sides of the tail sheath, and these may be termed the 

 caudal pores.' 



The figures of two recent Chimseroid egg-capsules are here repro- 

 duced from Dr. Bashford Dean's monograph (PI. XXVII, figs. 3 

 & 4), from which it will be seen that the dart-like shape is the same, 

 both in the recent capsules and in Palceoxyris and Vetacapsula, but 

 no differentiation in head-, trunk-, and tail-sheath can be made out 

 in the fossils. It is possible, however, that the ' beak ' of Palceoxyris 

 may be the equivalent of the opercular flap and its appendages. 



It is not suggested here that these fossils are the egg-cases of 

 actual Chimseroids. In fact, their spiral disposition suggests that 

 they have some affinities with those of the Cestracionts. But, if 

 Bashford Dean be right — and he brings forward some strong argu- 

 ments in favour of his theory 1 — in his view that the Chimseroids are 

 a highly specialized family, branching off from the Selachian stem 

 near, or even within, the group of Palaeozoic Cestracionts, and are not 

 the lowly survivors of a primitive stock from which the Selachians 

 arose (as has been formerly supposed) : then it is possible to look 

 upon these fossils as the capsules of a more generalized ancestor of 

 both the Cestracionts and the Chimseroids, which combined in their 

 egg-cases some of the characteristics of both. 



In conclusion I would here wish to express my thanks to Dr. E. 

 Kidston for the kindly help and advice which he has given me, and 

 the trouble that he has taken in reviewing this paper ; also for the 

 loan of specimens and carefully collected notes, which have been 

 invaluable for its production. I am further indebted to Mr. E. A. 

 Newell Arber, to Dr. H. H. Swinnerton, to Dr. P. Oswald, and to 

 Dr. "Walcot Gibson, for much help and advice. Also to Mr. E. 

 M. Mundy, of Shipley Manor, and to the manager of the Digby 

 Brickworks, for the facilities which they have granted me for 

 examining the ironstone nodules of their claypits. 



EXPLANATION OF PLATES XX1V-XXVII. 



Plate XXIV. 



Fig.l. Palceoxyris helicteroides (Morris). Meadow Lane Claypit, Alfreton. 

 Author's collection. Natural size. 



2. Palceoxyris helicteroides (Morris). Digby Claypit, Kimberley. Two- 



thirds of the natural size. Author's collection. 



3. Palceoxyris helicteroides (Morris). Digby Claypit, Kimberley. Two- 



thirds of the natural size. Showing parasitic bosses on the beak. 

 Author's collection. 



4. Palceoxyris helicteroides (Morris). Digby Claypit, Kimberley. Natural 



size. Showing the arrangement of segments in the beak. Author's 

 collection. 



5. Palceoxyris (Vetacapsula) johnsoni, Kidston . Two-thirds of the natural 



size. Author's collection. 



Op. cit. p. 155. 



