104 
THE GEOLOGIST. 
The socket-plates arc largo, and partially united to the lower portion of the car- 
dinal j)roccss. Under tiieso, on tlic bottom of the valve, may be seen the 
quadruple impressions left by the occlusor, and which occupy above one-third 
of the len£>'tli of the valve, and are ari'anged 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 apj)carcd to me (after a 
minute and lengthened exammation of a multitude of English, Irish, Scotch, 
and foreign examples of these said species) that they all appear so intimately 
connected and linlced together by intermediate and insensiljle 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, tihey 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 outline 
(fig. 17) will be found ia 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. 
Fig. 16 of our plate would agree with PliUlips' 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 uniisually great. 
In St. Kellii the dorsal valve is more than ordinarily convex, with a slight de- 
Ercssion along the middle, while the ventral valve, convex at or about the beak, 
eeomes concave towards the margin. The external sculpture varies likewise 
in different specimens, for it so happens that the larger radiating striae are at 
longitudinal stria;, while on the contrary, in other examples, in the mtervening 
spaces, there is room for two or more distinct but smaller longitudinal striae, 
and in this 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 
those intermediate shapes which unite St. senilis and St. radialis to St. crenis- 
tria, etc. 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 hmidi'ed and forty-one at Raes Gill. It is 
found also at Brockley and Middlcholm, near Lcsmahago ; Auchentibber, High 
Blantyre ; Robroyston, north of Glasraw ; and at PhilipshiU and Darnley- 
quarries, East Kilbride. In Renfrewshire, at BaiThead and Orchard-quarry, 
Thoruliebank. In Dumbartonshire, at Netherwood, near Castlccary. In Ayr- 
shire, at Roughwood and West Broadstone, Beith ; Auchenskeigh, near Dairy ; 
Goldcraig and Monkredding, Kilwinning ; Hallerhu'st, Stevenston ; Craigie, 
near Kilmarnock ; Neatliernewton and Moscow, parish of Loudon ; and Mea- 
dowfoot, near Drumclog ; also Cessuock, near Galston. In Stu'lingshire it is 
found in several stages. In the Balglass and Mill Burn beds, the Campsie 
main-limestone, and Corrie Burn beds. It occurs also in Arran and Bute. 
In Fifesliirc the varieties above enumerated, as well as St. Kellii have been 
found at Limekilns, above Queensferrv. 
times 
sufficient for a single smaller 
