7() CARBONIFEROUS LAMELLIBRANCHIATA. 



original description is "remarkably elongate, with a curved oblique convexity 

 from tlie beaks ; lines of groAvtli delicate, forming furrows on the convexity." 

 Unfortunately little can be learned from the figure, which is that of a young 

 example, and is less flattened at the sides anteriorly and more cylindrical than in 

 the adult, the posterior end being less expanded. M'Coy, while referring to the 

 occurrence of M. lingualis in Ireland, says " some obscure specimens of this shell 

 have occurred/' but gives no figures. He describes and figures a typical adult 

 example of what I consider to be Phillips's shell, under the name Lithodomus 

 dactyloides. Unfortunately the original of this figure has disappeared, but there is a 

 specimen in the Griffith Collection in the Museum of Science and Art, Dublin, 

 which bears the name L. dactyloides, and this I take to be the adult and nearly full- 

 grown form of M. lingualis. This specimen is certainly of the same species as the 

 shells figured PI. I, figs. 3 and 5, from Irish Carboniferous beds. M'Coy's 

 description is as follows : — " Width three times the greatest length, cylindrical, 

 extremities obtusely rounded ; beaks inconspicuous, close to the anterior end. Surface 

 marked by concentric lines of growth, which near the beak are decussated by very 

 fine, oblique, longitudinal strioa." It will be noted that in his figure the decussating 

 striae are shown over the greater part of the shell. 



The specimen from Park Hill, PI. I, figs. 4, 4 a, the original of Mr. R. 

 Etheridge's Modiola lithodomoides, I regard as the adult and full-grown shell of 

 Phillips's M. lingualis. This specimen is much larger than the shell figured by 

 Phillips, but equally fine specimens also occur at Castleton with examples of the 

 shell in various stages of growth. Most palseontologists have never given 

 sufficient attention to the changes in size and shape due to growth, but have 

 multiplied to a very great extent the number of species without any real excuse 

 for so doing. 



Mr. R. Etheridge, jun., gave a very perfect and accurate description of this 

 shell from the specimen which I am able to reproduce by the kindness of Sir A. 

 Geikie, it being in the Museum of the Geological Survey, Jermyn Street. 

 Mr. Etheridge, however, omits to point out the subangulated posterior margin, 

 and I think confounds two different forms under one title. The Scotch specimens 

 from the neighbourhood of Beith, Ayrshire, possessing certain well-marked 

 characters, which in ray opinion are of specific value, and the specific name of 

 lithodomoides must be retained for this shell. The Scotch shells are more 

 cylindrical, much smaller dorso-ventrally compared with the aiitero-posterior 

 diameter. The hinge-line is comparatively much louger than in L. litliodomoides 

 from English and Irish limestones, always equalling at least two-thirds of the 

 greatest length of the shell in extent ; and the elongated hollow trough formed by 

 the depression of the upper edges of the valves is only found in the Scotch forms. 



The second figure given by Mr. Etheridge shows the greater length of the 



