200 Prof. T. R. Jones and Dr. H. Woodward— 
on the outer edge, and on the inside bounded by oblique edges, two 
(below) meeting near the centre at an angle of about 25°, and two 
(above) at an angle of 60°. i 
This differs but little from the last-mentioned specimen, found in 
the Penwhapple Burn, Girvan, except in being larger and more 
symmetrical—that is, less distorted. Hence the two appear to 
belong to one and the same species; and their geological stages are 
not very far apart, for the Girvan species came from the Penkill 
Beds, as mentioned in the Table at pp. 235-6 of the “‘ Monograph ” 
alluded to above, which are referred to the ‘lower part of the 
Upper Silurian,”—that is, to the “Upper Llandovery or May Hill 
Sandstone,” see page 227 of the “Monograph.” 
This form is similar in essential particulars to Mr. J. M. Clarke’s 
Devonian Dipterocaris penne-Dedali (op. cit. p. 123, fig. 1); but 
differs from it in the two ale, taken together, having a rounded 
outline, and the posterior cleft is not so widely open. The British 
form is also much smaller. From D. Procue, Clarke, and D. pes- 
cerve, Clarke, it differs still more in several respects. 
Fig. 3 is a dark film, slightly thickened, and whiter on the outer 
curve of each valve, and at each end of the left valve; it also has 
some ferruginous thickenings at the centre and elsewhere. On a 
hard dark-grey shale, of Upper Silurian (Salopian) age, from 
Penyglog quarry, Corwen, on the border of Counties Merioneth 
and Denbigh, North Wales, Mr. G. J. Williams’ collection. 
Left ala of the Girvan 
specimen according to 
the figure. 
Greatest length of the right-hand moiety or ala... 28 mm. .. 12 mm. 
Greatest width _,, is 65 ae, 2s Gas wale 4. yy 
Length of the posterior cleft or notch... .... ... 15 4, 866 O\ <p 
Length of the anterior ,, ene = abouty » MG tay bei 43 ,, 
Median junctural interval ... ... ... about 6 ,, if Ly aieh 
4, Myrinorp Suutis. Plate X. Figs. 4a, b, c, and 5. 
These were selected some time ago as having an Hstherioid ap- 
pearance ; but, under the microscope, the evidence of such an alliance 
disappears. Our Figs. 4 and 5 approximate to some of the Silurian 
Mytili described and figured by Barrande in his “Syst. Silur. 
Bohéme,” vol. vi. such as pl. 210, figs. 1, 4, 5, and 6, M. consors, 
and fig. VI. 6, M. longior. 
Figs. 4a, b, c, represent a black film, somewhat crumpled and 
broken on the upper edge; it shows very delicate concentric lines 
of growth. On a hard black shale; British Museum, No. 48762; 
without a locality ; from the ‘ Bright Collection.” 
The valve is 6 mm. long, and 4mm. high. 
Fig. 5. A black film on dark-grey, hard shale, with ferruginous 
face. A small portion of the shell or test remains at the apex. 
British Museum, No. I. 2612. Probably from the Tremadoc Slates ; 
no locality. 
The valve is 20 mm. long, and 12 mm. high. 
The junction is obscure in the figure given of the similar fossil in Nicholson and 
Etheridge’s “‘ Monograph of the Silurian Fossils of the Girvan District in Ayrshire,” 
vol. 1. 1880, p. 212, pl. 14, fig. 21. 
