230 CARBONIFEROUS LAMELLIBRANCHIATA. 



growth, with here and there, towards the inferior margin, two or three deeper 

 concentric sulci. 



Dimensions. — Fig. 10, PL XVII, the type of Portlock's Amphidesma, 

 ax in iformis, measures — 



Antero-posteriorly. Dorso-ventrall}'. Elevation of valve. 



22 mm. 20 mm. 5 mm. 



From side to side. 



Fig. 19, PI. XVII, measures 30 mm. 27 mm. 15 mm. 



Localities. — England : ? Limestone, Lowick ; the Redesdale Ironstone shales, 

 Redesdale and Bellingham, Northumberland ; Black Limestone, Poolvash, Isle 

 of Man. Scotland : Upper Limestone series, Garngad Road, Glasgow ; Gare, 

 Carluke; Newfield, High Blantyre. Lower Limestone series in the Lingula 

 Ironstone ; Calmy and Main Limestones, Carluke ; Kirktouholm, Craigenglen ; 

 Langside, Beith. A marine band in the Calciferous-sandstone series, Fife. 

 Ireland : Limestone shales, Clogher, co. Tyrone. 



Observations. — The confusion as to the nomenclature of this species has been 

 very great, partly owing to the fact that Portlock gave names to several local 

 varieties of the same species, and was unfortunately mistaken as to their specific 

 identity. Under the name Venus ? carbonaria (p. 438) he remarks, "It is 

 probable that the fossils here noticed should be placed either in Mactra or 

 Amphidesma. Three forms occur closely approximating to each other ; and, as 

 the several specimens exhibit considerable variation, they may prove varieties of 

 the same species ; I, however, adopt the name Amphidesma. 



" 1st. Amphidesma carbonaria, Venus carbonaria ? Sow. — This is the variety 

 nearest to Sowerby's species. 



" 2nd. Amphidesma depressa. — The general form the same, but the truncation 

 is less marked and the shell less convex. 



"3rd. Amphidesma axiniformis. — More convex; beak rather nearer the front; 

 truncation more distinct; less transverse." 



A fourth form, A. deltoidea, the Cypricardia deltoidea of Phillips, is also described 

 (p. 439) ; but this is only the cast of the interior of the third form ; and Phillips's 

 shell came from the Petherwin beds of Devonshire. 



The references to V. carbonaria, Sow., and Cypricardia deltoidea, Phillips, were 

 unfortunately not quite correct. The former shell is Schizodus carbonarius, 

 PI. XVII, figs. 7—9 ; and the latter the Rev. G. F. Whidborne now refers to 

 Myophoria deltoidea (' Devonian Fauna, Pal. Soc.,' vol. iii, 1896, pt. 5, p. 93), which 

 is a totally different shell from Portlock's species. 



Of the four species invented by Portlock two only were new ; and the first of 

 these, " depressa" can hardly stand as the name of the species, because the shell 

 to which it was originally given is a varietal form, and not at all typical of the 



