942 DR F. R, C. REED ON THE 



Horizons. — (l) Balclatchie Group; (2) Stinchar Limestone Group? 



Localities. — (l) Ardmillan, Dow Hill, Balclatchie ; (2) Craighead. 



Remarks. — This small shell, of which some specimens from Ardmillan in 

 Mrs Gray's collection were labelled by Davidson Rh. cuneata, var. ardmillanensis 

 (MS.), is abundant in the Balclatchie Beds. In the Craighead limestone there is 

 a variety with a weak incipient sinus and coarser ribs. A shell much like it with 

 rather more numerous plications (24-26) was described by Davidson as Rh. 

 Weaveri ?, but it cannot be associated with the Silurian species of this name which 

 occurs at Tortworth, and most probably it is only a variety of Rh. ardmillanensis. 

 The first-named variety from Craighead was figured by Davidson as Rh. cuneata ? 

 (op. cit., pi. x, fig. 9), and has rather coarser larger and fewer ribs than the common 

 Ardmillan type, and it seems rather broader and less elongated ; but it can scarcely 

 be regarded as a distinct species. Rh. [Camarella] cuneatella, Davidson,* from the 

 Balclatchie conglomerate, has fewer and broader plications and they are only developed 

 near the margin (see p. 926). The typical Wenlock Rh. cuneata, Dalm., differs from 

 ours in possessing fewer and coarser ribs, a non-truncate anterior end, and clearer 

 traces of fold and sinus. Rh. ardmillanensis more resembles in general appearance 

 Camarophoria subcuneata, Hall,t from a much higher stratigraphical horizon. 



Rhynchotreta cuneata (Dalman), var. 

 (Plate XXIV, fig. 18.) 



1866. Rhynchonella cuneata, Dalman, Davidson, Mon. Brit. Foss. Brack., vol. iii, p. 164, pi. xxi, 

 figs. 7-12. 

 Non 1883. Rhynchonella cuneata, Dalman, Davidson, ibid., vol. v, Silur. Sup})/., p. 152, pi. x, figs. 9, 10. 



In this species Davidson states that there are 10-14 ribs, of which 4 or 5 lie in the 

 fold and 3 or 4 in the sinus. The Girvan examples sometimes have as few as 8, but 

 occasionally as many as 14 ribs, and there is no marked fold in the brachial valve or 

 sinus in the pedicle-valve. The shells also are flatter and have longer straighter 

 shoulders. It should also be noticed that there is always a marked interspace (but 

 not wider or deeper than those between the other ribs), down the median line of the 

 brachial valve, and that along it lies the median septum extending for half the length 

 of the shell. There is considerable variation in the degree of coarseness of the ribs. 



The form from the Lower Ordovician beds, attributed to this species by Davidson 

 (op. cit., p. 153), is now separated off as distinct under the name Rh. ardmillanensis. 

 Some small rare shells from Thraive Glen are very doubtfully referable to Rh. cuneata, 

 Dalman. 



Horizon. — (l) Mulloch Hill Group ; (2) Drummuck Group (Starfish Bed) '. 



Localities. — (l) Mulloch Hill, Craigens ; (2) Thraive Glen. 



* Davidson, op. cit, vol. v, Silur. Suppl., p. 200, pi. x, fig. 11. 

 t Hall and Clarke, op. cit, Brach., ii, pi. lxii, fig. 35. 



