SYNOPSIS OF GENERA. 
Ivii 
picion that a closer examination, of specimens of the various species described 
under the latter genus, may afford evidence of the structural features exhibited 
in Mesothyra. No simple bivalved carapace unquestionably referable to the genus 
Dithjrocaris, is known in the American Devonian ; the species Dithyrocaris 
Belli, Woodward, from the Middle Devonian of Gaspe, being too inperfectly 
known to allow its generic relations to be determined. To facilitate a com¬ 
parison of these two genera, the following index to the literature of the genus 
Dithyrocaris is given: 
Genus DITHYROCARIS, Scouler. 1843. 
1835. Argas, Scouler. Records of Science. 
1843. Dithyrocaris, Portlock, Geol. Rept. Londonderry. 
1844. Dithyrocaris, McCoy. Syn. Garb. Foss. Ireland. 
1849. Dithyrocaris, McCoy. Ann. Mag. Nat. Hist. 
1855. Dithyrocaris, McCoy. Syn. Biit. Pal. Fossils. 
1855. Dithyrocaris, F. A. Rcbmer. Beitr. iii zur Harzgeb. 
1863. Dithyrocaris, Salter. Quart. Jour. Geol. Society. 
1864. Dithyrocaris, Ludwig. Palseontographica, vol. xi. 
1865. Dithyrocaris, Salter and Woodward. Chart of Fossil Crustacea. 
1869. Dithyrocaris, Meek and Worthen. Pi-oc. Acad. Nat. Sci. Philadelphia. 
1870. Dithyrocaris, Woodward. Rept. Bi'it. Association. 
1871. Dithyrocaris, Woodward. Rept. Brit. Association. 
1871. Dithyrocaris, Woodward. Geol. Magazine. 
1873. Dithyrocaris, Meek and Worthen. Geol. and Pal. Illinois, vol. v. 
1873. Dithyrocaris, Woodward and Etheridge. Mem. Geol. Surv. Scotland. 
1873. Dithyrocaris, Woodward and Etheridge. Geol. Magazine. 
1874. Dithyrocaris, Woodward and Etheridge. Geol. Magazine. 
1878. Dithyrocaris, Kaysbr. JSlt. Devon. Ablag. Harz. 
1878. Rachura, Scudder. Proc. Boston Soc. Nat. History. 
1879. Dithyrocaris, Etheridge. Quart. Jour. Geol. Society. 
1882. Dithyrocaris, Packard. Amer. Naturalist. 
1883. Dithyrocaris, Packard. Monog. N. Amer. Phyllop. Crustacea. 
1884. Dithyrocaris, Clarke. Neues Jahrb. fiir Min. 
1884. Dithyrocaris, White. Thirteenth Ann. Rept. Geol. Surv. Indiana. 
1885. Dithyrocaris, Novak. Sitzungsb. bohm. Gesellsch. d. Wissenschaft. 
1887. Dithyrocaris, Etheridge, Woodward and Jones. Fifth Rept. Committee Foss. Phyllop. 
Diagnosis. Carapace sub-quadrate in outline, bivalvular, the valves being 
connected by a simple, straight hinge-line, and gaping at the anterior extremity, 
forming the rostral cleft. Rostrum unknown. Each valve bears a single, 
gently curved carina. Optic nodes generally conspicuous. Abdomen composed 
of ? segments of which but one is naked. Post-abdomen consisting of caudal 
plate and three caudal spines. 
