ALLORISMA MAXIMA 419 



Beushausen (op. cit.) compares the genera Allorisma and Leptodomus ; but he 

 does not seem to be aware that M'Coy took a totally different type for each of his 

 two descriptions of the latter genus, as I have shown antea, pp. 228 and 379. 

 Naturally there is, as I have shown above, a certain amount of similarity between 

 SanguinoJites (to which the second type of M'Coy's Leptodomus undoubtedly 

 belongs) and Allorisma. 



Some of the shells now referred to Allorisma have been occasionally referred to 

 Myacites, Schlotheim, which genus certainly has many characters in common 

 with Allorisma, especially in external characters, possessing concentric ridges and 

 grooves covered with minute granules ; and the same may be said of allied genera 

 — Arcomya, Pleuromya, and others. Allorisma differs, however, from its Jurassic 

 congeners in being much longer, more inequilateral, having a large lunule and 

 escutcheon, and an edentulous hinge, and not gaping so widely, if at all. It 

 is not at all improbable that the Paleozoic Allorismse were ancestors of the allied 

 tuberculated Jurassic Lamellibranchs. TelUnomorpha, however, has a much closer 

 resemblance to Jurassic forms. 



Allorisma maxima, Portlock, sp., 1843. Plate XLVII, figs. 5 — 7 a. 



Sanguinolaeia maxima, Portloch, 1843. Eep. Geol. Londonderry, p. 434, pi. 



xxxvi, figs. 1 a, b. 

 Sanguinolites clava, M-Coy, 1851. Ann. Mag. Nat. Hist., 2nd ser., vol. vii, 



p. 172. 



— MAxiMus, Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 223. 



— CLAVA, M'Coy, 1855. Brit. Pal. Eocks and Tossils, p. 504, p].3F, 



fig. 12. 

 Allouisma subcuneata, MeeTc and Hayden, 1858. Proe. Acad. Nat. Sci. Phila- 

 delphia, p. 2G3. 



— — — 1865. Smithsonian Contrib. Knowl., 



No. 172, Pal. Upper Missouri, 

 p. 37, pi. i, figs. 10 «, b. 



— — Hayden, 1872. Eeport U.S. Geol. Surv. Nebraska, 



p. 221, pi. ii, figs. 10 a, b. 



— — B iff sby, 1878. Thesaurus Devonico-Carbouifcrus, p. 29G. 

 Sangtjinolites MAXIMUS, Bigsby, 1878. Ibid., p. 313. 



— — Etheridge, 1888. Brit. Poss., pt. 1, Palaeozoic, p. 289. 



— CLAVA, Etheridge, 1888. Ibid., p. 289. 



— 8UBCUNEATUS, Miller, 1889. N. Amer. Geol. and Paljeontol., 



figs. 768, 769, p. 460. 



Specific Characters. — Shell above medium size, transversely elongate, slightly 

 arcuate, clavate, gibbose anteriorly, compressed and somewhat expanded pos- 



