14 Prof. W. King on some 



On the anterior edge, which slopes forward, a finely marked 

 lineated impression (d') is seen passing from one deltidial 

 ridge-scar to the other. The lines of this impression run 

 parallel with, and somewhat resemble, those which cross the 

 deltidium, so that the difference between the two is difficult to 

 majke out : indeed both might be readily confounded. 



The deltidium is a variable structure in Palliobranchs 

 generally ; and its modifications are far from being properly 

 understood. As regards the deltidium in the genus under 

 consideration, one circumstance is remarkable : it has been in 

 a great measure overlooked ; at least I can find little, or rather 

 no, notice of it in the writings of previous observers. It is 

 this oversight which led the late J. W. Salter to institute his 

 genus Lingulella, which he typified with the Cambrian Lin- 

 gula Davisii, under the belief that its " pedicle-groove " and 

 " hinge-area " do not characterize any species of the old 

 Bruguierian genus. Oholus and some apparently related 

 genera*, also the Trimerellids, are the only shells I am ac- 

 quainted with that have a structure resembling the deltidium 

 of Lingula. In the latter family it is a conspicuous object, 

 having attained a maximum development, which is equally 

 the case with the deltidial ridges. 



The pedicle (fig. 4, y), which is cylindrical, consists of two 

 concentric layers, the innermost of which (fig. 3, h) is muscular, 

 and the other (g) corneous. Near the proximal extremity it 

 becomes suddenly reduced in diameter, and at the same time 

 compressed into the form of an oval, the long axis of which cor- 

 responds to the width of the shell. At the reduced part it is 

 attached by one of the flattened faces of the corneous layer to the 

 deltidium (fig. 3, g, a), the transverse and longitudinal lines of 

 which are marks of its attachment. The anterior edge or ex- 

 tremity (fig. 3, d) of the same face is inserted immediately in 

 front of the deltidium — that is, along the border of the hinge- 

 slope, the faint lineated impression previously noticed being 

 produced by its insertion. The sides or angles of the anterior 

 edge are converted into tendinous lobes, which are individually 

 attached to each of the ridge-scars (e) : the pressure of the lobes 

 against the end of the deltidial ridges evidently gives rise to 

 the ridge-callosities. The corneous layer is not present on the 

 opposite face of the pedicle, but makes its appearance a little 

 within the opening of the beaks of the shell. 



The muscular layer (h) , passing in advance of the corneous 

 one under the form of a much compressed cylinder, is rooted 

 in the narrow space between the hinge-slope and the attach- 



* Discina may be included, as its deltidium, although remarkably 

 modified, appears to agree with that of Lingula. 



