350 Prof. T. R. Jones and Dr. H. B. Holl on the 



constricted or bipartite, and even divided into distinct lobes 

 (vars. torosa, suhtorosa). In one instance, whether a variety 

 or a species is doubtful (fig. 12, pi. vi. Ann. & Mag. Nat. Hist. 

 1855, vol. xvi.), the front lobe is tripartite and the hind lobe 

 (obscure in the specimen) seems to be undivided. It is 

 difficult in very many cases to determine where varietal 

 changes end and specific distinctions take their stand. 



BeyricMa Kloedeni^ M'Coy, is subject to considerable 

 variation in the relative shape and proportions of its lobes, 

 ridges, and furrows, besides being affected by a subdivision of 

 the lobes, as in the varieties suhtorosa and torosa. In the nar- 

 rowness of the lobes and width of the furrows the varieties 

 intermedia sind pauperata a.ve conspicuous. In the latter the 

 furrows are very much widened at the expense of the ridges, 

 which are not only narrow but much attenuated below, a thin 

 line only partially connecting the front and hind lobes along 

 the ventral margin, .while the middle lobe is greatly reduced, 

 comma-like, and isolated. This form accompanies var. torosa 

 in the Upper-Ludlow rock of Aymestry Common (both as 

 casts), and is figured in the Pal. Biv. Entom., Proc. Geol. 

 Assoc. 1869, p. 14, fig. 8. 



Several British varieties are noticed (with references) in 

 the Geol. Mag. [1. c.) as being marked by the relative deve- 

 lopment of the three lobes, ranging from. the variety jpaupe- 

 rata^ having very narrow lobes, and the var. torosa, with its 

 subdivided lobes, to the typical Klcedeni, with well-developed 

 neatly-shaped lobes, and its var. tuherculata, in which the 

 lobes take up a large portion of the valves, and, leaving the 

 middle lobe more or less distinct, have become almost or quite 

 confluent below. A general tuberculation of the surface is 

 common in the last variety and is present also in some others. 

 In the collections made by Messrs. Smith and Vine we meet 

 with some of the varieties here alluded to, and with inter- 

 mediate conditions, whether of individual growth or of 

 progressive development. As B. Klozdeni varies in two 

 directions (in one set of individuals the lobes becoming 

 narrower and wider apart, and in the other becoming thick 

 and closer) , it is difficult to take all its varieties in one order. 

 We first take the typical forms, and then those showing- 

 attenuation of the lobes. 



I. Lohes free and plump. 



(1) Beyrichia Klcedeni, var. granulata, Jones. 



(PI. XII. %. 2.) 



Proportions :— L. 18. H. 11. 



The common three-lobed Beyrichia somewhat granulated 



