part 2] s yri ngoth rm s AjStj spirifeiuxa. 219 



Silurian genus Delthyris (for instance, D. elevata); but, whether 

 ibis is a case of homceomorphy, or whether the Silurian and Car- 

 boniferous species are genetically connected by a gradual transition 

 during Devonian times to the punctate condition seen in the 

 Carboniferous shells, cannot at present be determined. 



Spiriferina pekpltcata, sp. liov. ( Text-tig. 5 C, p. 211; 

 PL XIII, figs. 7 a-1 c & 10. ) 



Description. — Shell spiriferoid, of medium size, greatest width 

 at the hinge-line, cardinal extremities nearty right angles, or 

 slightly produced. 



Pedicle- valve subpyramidal ; cardinal area well defined, about 

 a third as high as wide, flat near the hinge-line, but concave 

 towards the beak, which is small and pointed ; apical angle 120° ; 

 delthyrial angle 40° ; lateral slopes tumid and evenly convex ; sinus 

 angular, about twice as wide as deep. 



Brachial valve evenly convex, mesial fold angular, and 

 about as wide at the margin as two of the adjacent costse and the 

 furrow between them. 



Each lateral slope in both valves bears about six angular eostaj, 

 separated by equally angular furrows slightly narrower than the 

 costse. 



Surface of both valves, except the area, crossed by concentric 

 scale-like lamina?, not evenly distributed over the surface of the 

 shell. Shell-substance coarsely punctate. 



Delthyrial supporting-plates and median septum as in Sp. octo- 

 plicata. 



The dimensions of an average specimen are as follows : — Width 

 along hinge-line, 22 mm.; length of brachial valve, 10 mm. ; height 

 of area, 6 mm. 



Holotype. — A specimen in the collection of Mr. J. W. Jackson, 

 P.Gr.S., of Manchester (figured in PL XIII, figs. 7 a-1 c). 



Horizons and localities. — Carboniferous Limestone; of the 

 holotype, Upper Dibunophyllwn Zone, Peak's Hill. The typical 

 form occurs only in the Dibit nophyllum Zone. 



Comparison with Sp. octoplicata, etc. — This species differs 

 from Sp.. octoplicata in having a more clearly defined and less curved 

 area, and in the greater number of its costse, which are separated 

 by narrower and shallower furrows than in that species. 



From both Tylothyris laminosa and Punctospirifer scabricosta 

 it differs in the relative uniformity of the size of its costse and the 

 fold in the brachial valve, and it maybe further distinguished from 

 the former species by its punctate shell-structure. 



One of the specimens figured hj Sowerbyas Spirifer octoplicata, 

 namely [ 32] pi. dlxii, fig. 4, and refigured by Davidson, 1 in all proba- 

 bility belongs to this species, although its fold is of an exceptional 

 character, being divided anteriorly into three by two short furrows. 1 



1 Davidson [4] pi. vii, fig. 37; pi. lii, fig-. 13. The original specimen is 

 distorted, and Davidson's figures are idealized. 



