﻿106 



BRITISH CARBONIFEROUS BRACIIIOPODA. 



ventral one ; beak small, much incurved, sometimes inconspicuous, on account of the 

 gibbosity of the dorsal valve ; sinus moderately deep, fold more or less prominent. The 

 surface of each valve is ornamented by from fifteen to forty ribs, those on the fold and 

 sinus being generally larger and wider than those that cover the lateral portions of the 

 valves ; the ribs also become more numerous by intercalation or bifurcation at various 

 distances from the beak and umbone, and especially so on the lateral portions of the valves, 

 where they are generally much curved and smaller. Dimensions very variable ; three 

 specimens measured — 



Length 10, width 13, depth 8 lines. 

 >, 8, ,, 10, ,, 7 ,, 

 7 9 6 



Obs. After a long and minute examination of the typical and many other examples 

 of Terebratula jlexistria and T tumida (Phillips), 1 as well as of Hemithyris heteroplycha 

 (M'Coy), I arrived at the conclusi .1 that they were all variations in shape of a single 

 species, for which the designation of Jlexistria has been retained. At p. 222 of the 

 ' Geology of Yorkshire' (vol. ii), Professor Phillips describes his two shells with the fol- 

 lowing words : " Terebratula jlexistria. Oblate, depressed, mesial elevation rounded ; lower 

 valve smaller, flatter, with inconspicuous beak ; many obtuse striae, much curved on the 

 sides." 



" Terebratula tumida. Oblate, tumid, lower valve flatter, with inconspicuous beak ; 

 striae strong and rounded on the middle, smaller and curved on the sides." Both are 

 stated to be from Bolland, and I have reproduced in PI. XXIV the author's original 

 figures (I and 7), also those given by Professor M'Coy of his H. heteroplycha (fig. 5). 

 Every intermediate passage or gradation of shape and character can be found, whereby 

 the three shells above mentioned are intimately connected ; in some the ribs on the fold 

 and sinus are few in number, large, simple, and varying from three to six ; those on the 

 lateral portions of the valves being smaller and almost entirely simple ; while in other 

 examples, some or all the ribs have bifurcated or augmented by the means of inter- 

 calations at various distances from the beak and umbone, the relative disproportion in size 

 of the ribs on the fold and lateral portions of the valves not being so apparent. The beak 

 likewise is inconspicuous in some specimens, while in others it is slightly produced above 

 the umbone of the dorsal valve. In PI. XXIV, I have endeavoured to represent all these 

 appearances or variations of detail, to which the reader can refer. The umbone varies 

 also much in degree of convexity, being uniformly rounded in some specimens, while 

 there exists in others a slight median depression. 



Rh. jlexistria audits varieties maybe distinguished from Rh. pleurodon (to which some 

 examples approach) by their general shape, disposition, and character of their dichotomising 



1 The original examples of Professor Phillips's species form part of the Gilbertsonian Collection in 

 the British Museum. Professor M'Coy's H. heteroplycha is preserved in the Geological Museum at 

 Cambridge. 



