58 CARBONIFEROUS LAMELLIBRANCHIATA. 



the shell (which is shown as a groove in casts). This is a feature not usually met 

 with in the genus Modiola, but is seen in other Carboniferous bivalves, e. g. the 

 Avicula Hendersoni of Robert Etheridge, jun. M'Coy's Modiola divisa also had 

 this peculiar structure, and the specific name was given to it on that account ; but 

 I have, however, stated above that I consider it probable that these two forms 

 will be found to be the same. 



Only four Carboniferous shells have been referred to Modiola by American 

 palaeontologists, two of which, in a former Monograph, I thought probably belonged 

 to Naiadites. 



Many of the elongate, narrow, Modioliform shells occurring in the Jurassic 

 beds agree rather in shape with Lithodomus than with Modiola, to which they have 

 always been referred, it having been the custom to place all such shells in the 

 latter genus. Probably this has been due to the desire to establish the rock-boring 

 habits of a shell before classifying it with a group in which that mode of living is 

 characteristic. Habits, however, are a matter of environment and evolution, of 

 course depending on anatomical construction, but should not, I think, be used as 

 a basis for classification. 



Modiola patula, M'Coy, 1844. Plate I, figs. I, 2, 2 a, 2 b. 



Modiola patula, M'Coy, 1844, Synopsis of the Characters of the Carboniferous 



Fossils of Ireland, p. 75, pi. xiii, fig. 13. 

 Mttilus patulus, d'Orhigny, 1850. Prodrome de Paleontologie, vol. i, p. 134. 

 ? — PEENELLA, de Byckholt, 1847. Melanges paleontologiques, part 1, p. 144, 



pi. viii, figs. 20, 21. 

 Modiola patula, Bigshy, 1878. Thesaurus Devonico-Carbouiferus, p. 309. 

 — — B. Etheridge, 1888. British Fossils, vol. i, Palseoz., p. 285. 



Specific Characters. — Shell transversely elongate, obliquely spathulate, obliquely 

 gibbose. The anterior end is almost obsolete and obtusely pointed. The lower 

 margin, convex in outline at first, soon becomes slightly concave, until the oblique 

 convexity of the shell meets it, when it again becomes convex downwards. The 

 posterior border is obtusely rounded, and there is no angle at any part of its 

 extent, but it meets the lower margin with a more obtuse curve than it makes 

 above with the superior border. The hinge-line is ahnost straight, and equal in 

 length to almost one-half the extreme antero-posterior measurement of the shell ; 

 posterior to the hinge the upper border gently curves downwards, and passes 

 iinperceptiblyinto the posterior. 



The umbones are anterior but not quite terminal, small, remote from the 



