SYNOPSIS OF GENERA. lvii 



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 

 Dithyrocaris, is known in the American Devonian ; the species Dilhyrocaris 

 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. Aryas, Scouler. Records of Science. 



1843. Dithyrocaris, Poktlock, Geol. Rept. Londonderry. 



184-1. Dithyrocaris, McCoy. Syn. Carb. Foss. Ireland. 



1849. Dithyrocaris, McCoy. Ann. Mag. Nat. Hist. 



1855. Dithyrocaris, McCoy. Syn. Brit. Pal. Fossils. 



1S55. Dithyrocaris, F. A. Rce.mer. Beitr. iii zur Harzgeb. 



18(53. Dithyrocaris, Salter. Quart. Jour. Geol. Society. 



Im'iI. Dithyrocaris, Ludwig. Pahvontographica, vol. xi. 



1865. Dithyrocaris, Salter and Woodward. Chart of Fossil Crustacea. 



1869. Dithyrocaris, Meek and Worthen. Proe. Acad. Nat. Sci. Philadelphia. 



1S7U Dithyrocaris, Woodward. Rept. Brit. Association. 



1871. Dithyrocaris, Woodward. Rept. Brit. Association. 



1871. Dithyrocaris, Woodward. Geol. Magazine. 



1873. Dilhyrocaris, Meek and Worthen. Geol. and Pal. Illinois, vol. v. 



1873. Dithyrocaris, Woodward and Etheridoe. Mem. Geol. Surv. Scotland. 



1873. Dithyrocaris, Woodward ami Etheridge. Geol. Magazine. 



1874. Dithyrocaris, Woodward and Etheridge. Geol. Magazine. 

 is" 1 -. Dithyrocaris, Kayser. JElt. Devon. Ablag. Harz. 



1S78. 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. fur Min. 



1884. Dithyrocaris, White. Thirteenth Ann. Rept. Geol. Surv. Indiana. 



1885. Dithyrocaris, Novak. Sitzungsb. bohm, Gesellsch. d. Wissenschaf't. 



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. 



