part 4] 



CAMBRIAN HORIZONS OF COMLET. 



851 



portion of the Black Limestone of Comley, now called the 

 Lapioortliella Limestone. 



The ventral valve is conical and nearly circular in outline, some- 

 what flattened posteriorly, and has a very slight indentation at the 

 middle line ; the posterior slope may he vertical (fig. 12), or inclined 

 backwards (fig. 11), or a little forwards (fig. 9 a) ; the anterior and 

 lateral slopes are nearly straight ; the false area is ill-defined and 

 traversed hy a vertical furrow, indistinct towards the margin. The 

 internal marks vary with the amount of exfoliation. Fig. 9a shows 

 the cast of an interior where slight exfoliation has occurred; fig. 9 b 

 represents the same specimen after the material of the apical 

 callosity had been removed. It discloses the cast of the elongated 

 visceral cavity and those of the vascular sinuses and cardinal 

 muscle- scars. Fig. 10 shows an interior in which the visceral 

 cavity and the two little pits are well seen. The foramen itself is 

 lost in the broken posterior margin, but the narrowing of the 

 visceral cavity to the notch or groove connecting it with the 

 foramen is quite obvious. The casts of the little pits are rarely or 

 never seen ; they are not obvious, as in A. comleyensis. 



Occasionally the easts of the vascular sinuses are proportionally 

 wider apart than they are in the more typical form represented in 

 fig. 9 b. 



The dorsal valve is very little wider than long, rather strongly 

 convex, and has the apex bent down almost to the margin. In the 

 interior (figs. 15 & 17) there is a septal ridge which widens at its 

 termination ; the vascular sinuses are well developed posteriorly ; 

 between them and the septal ridge are two elongated prominences ; 

 the cardinal muscle-scars are large. 



The shell-substance is corneous and very thick, both in the dorsal 

 valve (see fig. 14 b) and in the apical callosity of the ventral. The 

 exterior is marked by concentric striae, fairly closely set and 

 apparently strongest on the dorsal valves ; where exfoliation has 

 occurred, radiating striae are clearly seen. 



Dimensions. — The lengths vary from 1"5 to 3 mm. ; the pro- 

 portion of length to width is about 1 to IT ; and that of length to 

 height is 1 to "4 in ventral, and 1 to "25 or "3 in dorsal valves. 



Localities and horizon. — Comley, Lower Cambrian, from 

 the Lapworthella Limestone of Comley Quarry, horizon Ad; also 

 near Neves" Castle, in the Wrekin district. 



Comparison between Acrothira comletensis and A. cf. sera. 



(1) Posterior margin . . 



(2) Inclination of pos- 



terior slope 



(3) Concentric strife ... 



(4) Radiating strise ... 



(5) Visceral cavity ... 



(6) Casts of little pits 



(7) Septal ridge 

 dorsal valve 



of 



A. comleyensis. 

 decidedly notched, 

 forwards. 



weak and very close- set. 

 barely visible, 

 widely bell- shaped, 

 very obvious. 



{strong posteriorly, 

 weak centrally, 

 reaching far forwards. 



A. cf. sera. 



almost straight. 

 f forwards, vertical or 

 L backwards. 



comparatively strong. 



moderately strong. 



elongate. 



not obvious. 



[ weak posteriorly, 

 widening centrally. 



