112 



BRITISH SILURIAN BRACHIOPODA. 



Spec. Char. Ovate, longer than wide ; valves almost equally and regularly convex ; 

 no regular fold or sinus, but a slight elevation is perceptible near the front in the dorsal 

 valve ; sides rounded, and narrowed towards the front, which is also either straight or 

 gently rounded. Beak more or less incurved, with a small triangular opening visible 

 under its angular extremity ; but this is often concealed by the incurvation of the beak. 

 Surface smooth, marked only with a few concentric lines of growth. Shell-structure 

 impunctate. Two specimens measured — 



Length 9, width 6, depth 4^ lines. 

 J) 9, „ 7, „ 5 „ 



Obs. Prof. M'Coy was the first to describe this species ; but he was in error when 

 placing it in d'Orbigny's genus Hemithyris, which is itself no more than a synonym of 

 Bliynchonella ; for, as may be seen in a specimen discovered by Mr. J. Thomson (fig. 25), 

 the shell was provided with spiral processes, and is a Meristella. Nor could it, for the 

 same reason, be classed with Terehroiula. In certain localities it abounds in the condi- 

 tion of internal casts, which, in Prof. M'Coy's words, " show in the entering [dorsal] 

 valve, two sub-parallel, approximate, longitudinal sulci, marking the inner [outer] edges 

 of the muscular impressions, and with a fainter sulcus between them, left by the slight 

 mesial septum ; two pits near the beak, left by the apophyses, strong. Receiving 

 [ventral] valve M'ith two strong dental lamellae, one each side of the beak, and a slight 

 indication of a mesial septum : a few straight, once- or twice-branched impressions of the 

 pallial vessels on each side." The shell is usually of an elongated oval form ; but some 

 examples are almost circular, and even occasionally, though rarely, a little wider than 

 long. I cannot consider Prof. Schmidt correct in placing M. angustifrons, M'Coy, among 

 the synonyms of Atrypa cassidea, Dalman (Silurische Formation von Ehstland, Nord- 

 Livland und Oesel; Archiv, &c., 1858, p. 209). 



Position and Locality. At p. 230 of ' Siluria' (1859) this fossil is quoted by Salter 

 as one of the characteristic fossils of the Lower Llandovery rocks. In Scotland it occm-s 

 by millions in a greenish and rusty-coloured sandstone at Mullock Hill, Dalquharran, 

 Craighead; as well as in other localities of the Girvan Valley, Ayrshire. Mr. Salter 

 traced this species in the Llandovery formation in South Wales, along the borders of 

 South Wales, at Rhayader, Cilgwyn, and Pen-y-lan, near Llandovery ; also near Builth. 



Meristella didyma, Dalman (sp.). PI. XII, figs. 1 — 10. 



Teeebratula ? DIDYMA, Dalman. K. Vet. Akad. Handl., p. 146, pi. vi, fig. 7, 1828. 

 Atrypa — Ilisinger. Letbsea Suecica, p. 77, pi. xxii, figs. 7 a, b, c, 



1837. 



Tebebeatula sacculus, Fon Buck. Ueber Terebrat. ; Akad. Berlin, 1834, p. 90 (not 

 T. sacculus, Martin). 



