104 



THE GEOLOGIST. 



The socket-plates are large, and partially united to the lower portion of the car- 

 dinal process. Under these, on the bottom of the valve, may be seen the 

 quadruple impressions left by the occlusor, and which occupy above one-third 

 of the length of the valve, and are arranged in paii-s, divided by a short rounded 

 median ridge. 



Many so-termed species appear to have been created out of this most variable 

 species, and indeed it is most puzzling and difficult to say how far we may be 

 permitted to limit the extent of variation, but it has appeared to me (after a 

 minute and lengthened examination of a multitude of English, Irish, Scotch, 

 and foreign examples of these said species) that they all appear so intimately 

 connected and linked together by intermediate and insensible graduations of 

 shape, that one would not be justified, I think, in maintaining as distinct what 

 ought in reality to be united. In Scotland we find examples which might be 

 referred to the Strep, crenistria, St. arachnoideus, St. senilis, St. radialis, and 

 St. Kellii, and of most of which representations are given in our plate ; but 

 considering St. crenistria as the typical shape, if any of the other names are 

 retained, they should be so simply as varietal designations. St. crenistria has 

 sometimes attained considerable dimensions, a Scottish example having mea- 

 sured three inches in length by four and a-half in width, and of this an outHne 

 (fig. 17) will be found in our plate, and the British Museum possesses a Bel- 

 gian example of still larger proportions. It is also a Devonian as well as a 

 Carboniferous species. 



Eig. 16 of our plate would agree with Phillips' original typical form of St. 

 crenistria. St. arachnoideus is a still more depressed condition of the species, 

 while in the variety senilis the convexity of the ventral valve is unusually great. 

 In St. Kellii the dorsal valve is more than ordinarily convex, with a slight de- 

 pression along the middle, while the ventral valve, convex at or about the beak, 

 becomes concave towards the margin. The external sculptui-e varies likewise 

 in different specimens, for it so happens that the larger radiating striae are at 

 times so close that there exists hardly space sufficient for a smgle smaller 

 longitudinal strife, while on the contrary, in other examples, in the intervening 

 spaces, there is room for two or more distinct but smaller longitudinal striae, 

 and in tliis last category will be found located the S. radialis. If, there- 

 fore, we are to maintain as distinct the so-termed species enumerated 

 at the commencement of this description, it will be necessary to create as many 

 more, for it will be often most puzzling to know where to locate several of 

 tliose mtennediate shapes which unite St. senilis and St. radialis to St. crenis- 

 tria, clc. The varieties crenistria and radialis were figured by David Ure in 

 his " History of Rutherglen and Kilbride 1793. 



St. crenistria and its varieties radialis and senilis occur in many Scottish 

 localities. At. Gare in Lanarkshire it occurs at two hundred and thirty-nine 

 fathoms below "EU coal," and three himdi-ed and forty-one at Eaes Gill. It is 

 found also at Brockley and Middleholm, near Lesmahago Auchentibber, High 

 Blantyre; Kobroyston, north of Glasgow; and at PhilipshiU and Darnley- 

 qunrnes. East Kilbride. In Renfrewshire, at BaiThead and Orchard-quaiT}^ 

 Tliornliebank. In Dumbartonsliire, at Netherwood, near Castlecary. In Ayr- 

 shire, at Boughwood and West Broadstone, Beith ; Auchenskeigh, near Dairy ; 

 Goldoraig and .Mi)nkrcdding\ Kilwiuning; Hallerliirst, Stevenston; Craigie, 

 near Kilmarnock ; NrailionicwUMi and Moscow, parish of Loudon; and Mea- 

 dow foot, near Druniclo^'; al>o Ccs^^noek, near Galston. In Stii-lingshire it is 

 fouud in several stages. In the Balglass and Mill Buiii beds, the Campsie 

 maiu-lnuestone. and Corric Burn beds. It occiu's also in Arran and Bute. 

 In Fifesliiro the varieties above enumerated, as well as St. Kellii have been 

 fouud at Liuiekilus, above Queensferrv. 



