72 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



Diagnosis. Shell transverse ; wider than long, more or less spherical ; the smaller 

 valve convex, even gibbous, attaining its greatest height before reaching the middle of the 

 valve ; the plaits on the mesial fold do not project much above the lateral ones. Larger 

 valve convex, exhibiting a wide sinus more or less defined, and extending to the front ; the 

 margin forming a convex line in that portion. The beak is small and acute, under which, 

 especially in young shells, the foramen is visible, almost surrounded by the deltidial plates, 

 except in a small portion, which is completed by the umbo ; in adult shells the beak 

 becomes so much recurved, that hardly any space remains between it and the umbo for the 

 passage of the peduncular fibres ; surface of valves ornamented by a variable number of 

 plaits of greater or less depth, not increasing much in width as they proceed from the beak 

 and umbo to the front; sometimes they are seen to bifurcate, but generally this appearance 

 is more due to the interposition of a new plait between the regular costae at variable 

 distances, than to true bifurcation : from distance to distance along the ridge of each plait 

 are seen to proceed long slender tubular spines, arising from an expanded base, and at 

 times exceeding six or nine lines in length ; their number is very variable, as well as the 

 regularity of their disposition. 



Length 13, width 15, depth 11 lines. 



05s. This is a well-known inferior Oolite shell, figured but not named by Knorr, 1 in 

 1755, by Walch and Knorr in 1768, 2 and by Captain Walcott in 1799. 3 It varies so 

 much in the number, width, and depth of its plaits, as to have tempted some authors to 

 divide them into more than one species. In some specimens I have counted forty-six plaits 

 on each valve, while others offer fewer, sometimes only twenty; to this variety M. D'Orbigny 

 has given the name of costata.* 



I do not agree with that author in placing this shell in the genus Hemiiliyris, from 

 the supposition that it had no deltidial plates, which is a mistake; nor do I admit that 

 the genus Hemithyris is in itself distinct from the Rhynehonella, as even in M.psittacea the 

 deltidium is visible, although in a less extended shape. 5 



1 Lapides Diluvii Universalis, tab. b iv, fig. 4. 2 Die Naturgeschichte der Verst., id. 



3 Descriptions and Figures of Petrifactions found near Bath, fig. 31. 



4 Prodrome, vol. i, p. 286. 



5 On reading over the article Hemithiris, Pal. Franc., Ter. Cretacees, vol. iv, p. 342, 1847, M. D'Orbigny 

 states : " This genus is very nearly related to Rhynehonella, and is only distinguished by its foramen 

 contiguous to the hinge, and no deltidium. We believe that two distinct genera may be established in this 

 division. The name of Hemithiris may be preserved for those species with a fibrous texture, and without 

 pores or exterior spines, the genus thus circumscribed with the recent form (H. psittacea) would number 

 17 species, the first are silurian .... to the species, likewise, of fibrous texture, but provided with 

 tubular spines, scattered or in lines, we will apply the name of Acanthothiris. We place in this the 

 Acanthothiris spinosa, D'Orbigny ; and costata, D'Orbigny ; and senticosa, inscribed in our 'Prodrome' under 

 the name of Hemithiris." But, as may be seen by referring to the article, Rhynehonella, Part I, I cannot 

 admit the three genera proposed for shells all similarly organised. Besides which, M. D'Orbigny places, 

 in the genus Rhynehonella, many species, the deltidium of which is not tubular, and species having the beak 



