LITHODOMUS LITHODOMOIDES. 79 



Dimensions. — The specimen, PI. I, fig. 6, from Beith, in the collection of 

 Mr. James Neilson, of Glasgow, measures — 



Antero-posteriorly . . . .83 mm. 



Dorso-ventrally at termination of hinge-line . . 23 mm. 



Laterally . . . . .20 mm. 



Long. Dorso-ventrally. Laterally. 



Another specimen measured 98 mm., 28 mm., and 20 mm. 

 Fig. 7, PI. I „ 87 mm., 21 mm., 19 mm. 



Localities. — Carboniferous shale of Lower Limestone of Beith, Ayrshire; 

 Loraburn, Glen Lora, and Lochwinnoch, Renfrewshire, from shale in the Lower 

 Carboniferous Limestone group. 



Observations. — Originally described, with a specimen from Park Hill, near 

 Longnor, Derbyshire, as M. lithodomoides by Mr. R. Bthridge, jun. I have 

 thought it well to separate the Scotch specimens, as they possess certain distinctive 

 characters, such as a comparatively much longer hinge-line, greater gibbosity 

 compared to the dorso-ventral measurement ; a deep, elongated hollow in each 

 valve between the edge of the hinge and the umbones and ridge proceeding 

 backwards from them ; and they do not possess the typical posterior border of the 

 Park Hill specimen. I have been enabled by the kindness of Messrs. Craig, 

 of Beith, John Smith, of Kilwinning, James Neilson, Dr. John Young, of Glasgow, 

 and Dr. Hunter, of Braidwood, to examine a large number of specimens of these 

 shells ; and though there is, as might have been expected, a certain amount of 

 variation of form, yet the general charactres and contour are fairly regular. The 

 distribution would appear to be very local, both horizontally and vertically. As far 

 as I can gather, this species is only met with in shale connected with the lowest 

 (Howrat, Auchenskeith, or Beith) Carboniferous Limestone in Ayrshire and 

 Renfrewshire. 



This shell is alluded to at page 43 of the Explanation of Sheet 22, 'Mem. 

 Geol. Survey, Scotland,' under the name " Lithodomus dactijloides, M'Coy," as was 

 pointed out by Mr. R. Btheridge in the original paper on this shell; a reference 

 which he considered erroneous, from the fact that the radiating lines of that 

 shell described by M'Coy are not present in the species under discussion. 



This species possesses one very marked and peculiar character in the elongated 

 hollow depression between the edge of the valves and the umbonal ridge, well 

 shown in fig. 8, PI. I, and to an extent in fig. 6 a, PI. I, which forms an 

 escutcheon, a character absent in other species of Lithodomus and in Modiola. 

 It is unfortunate that this shell always seems to occur with the test well preserved, 

 and no indications are to be seen of what conditions obtained within. The 

 ligament appears to have been marginal and inconspicuous. 



