ARCID.E. 127 



Psammobia, Etheridge, 1885. Brit. Foss., part 1, Palaeoz., p. 289. 



Macbodon, Hall, 1885. Pal. New York, vol. v, pt. 1, No. 2, pp. xxviii and 349. 



— Miller, 1889. N. Amer. Geol. Pal., p. 487. 



— Worthen, 1890. Geol. Surv. Illinois, pt. 8, p. 123. 

 Paballelodok, Whidbome, 1892. Monogr. Devonian Fauna (Pal. Soc), vol. ii, 



pt. 1, p. 39. 

 Macbodok, Keys, 1894. Geol. Surv. Missouri, vol. v, pt. 2, p. 120. 

 Macbodtjs, Tornquist, 1896. Fossil-fiihr. Untercarbon in den Siidvogesen, p. 84. 



Generic Characters. — Shell subrhomboidal, with a compressed dorsal slope, 

 which is bounded below by a ridge or rounded swelling, expanded posteriorly ; 

 borders, as a rule, subparallel. Hinge consisting of several small oblique teeth 

 in front, and a few long subparallel teeth behind, not extending the whole 

 length of the shell. A ligamental area is always present. Surface, even in the 

 smoothest forms, shows transverse strise with, as a general rule, radiating striae, 

 at least on the dorsal slope. 



Observations. — The name Parallelodon was first brought into use by de Koninck, 

 though he assigns the origin of the term to Meek and Worthen. As a matter of 

 fact, the name was invented by these authors, but never adopted by them ; and 

 it is evident that they did not recognise that the Carboniferous Arciform shells, 

 though closely allied to Lycett's Macrodon of the Lias, were distinct from it. In 

 a note to the description of Macrodon tenuistriatus (op. supra cit.) Messrs. Meek 

 and Worthen expressed themselves as follows : — " Should it be thought desirable 

 to change the name of this extinct genus, in consequence of the fact that Miiller 

 had used it for a genus of fishes in 1842, it might be called Parallelodon, from its 

 elongated parallel posterior teeth." They do not seem to have adopted this term 

 themselves ; but, in common with all American authors, use the name Macrodon, 

 Lycett, for the Arciform shells of the Devonian-Carboniferous period. 



The following is the description by Lycett of his new genus Macrodon, at p. 98 

 of Murchison's ' Outline of the Geology of the Neighbourhood of Cheltenham,' 

 1845 : — " General figure that of Byssoarca ; umbones small, situated near the ante- 

 rior extremity of the shell, and separated from each other by an area, an hiatus in the 

 ventral margin for the passage of a byssus, producing a corrugation in the edge of 

 the shell bordering it. Hinge-line straight and extended ; that of right valve has 

 six oblique parallel teeth placed at its anterior extremity, the innermost tooth or 

 posterior one extending the whole length of the hinge-line. The left valve is 

 destitute of teeth, but has fissures to receive the teeth of the other valve. The mus- 

 cular impressions closely resemble those of Cucullxa, the anterior having a sharp 

 prominent ledge projecting from the side of the shell ; the posterior one expanded 

 and indistinct." It will be noticed that the genus Macrodon has not the diverging 

 subparallel elongate teeth of Parallelodon, and that the anterior teeth are fewer in 



