4 bulletin: museum of comparative zoology. 



lobus Eichwald 1860, Panderia Volborth 1863, lUaenurus Hall 1863, 

 Illaenopsis Salter 1S66, Ectillaenus, Hydrolaenus, Octillaenus Salter, 

 1867, and Illaenoides Weller 1907. Certain other genera, such as 

 Symphysurus and Nileus have often been placed with lUaenus, but 

 they have more generally been recognized as belonging to the Asa- 

 phidae. The fundamental difference between the lUaenidae and the 

 Asaphidae is, of course, the presence in the former family of an 

 epistoma, and its absence in the latter. The absence of the grooves 

 on the pleura of the thorax in the Illaenidae and their very general 

 presence in the Asaphidae affords quite a safe criterion for judging 

 of the dorsal surface. 



The genera enumerated above may be taken up in order. 



Cryptonymus Eichwald, 1825. 



Observations geognostico-zoologicae per Ingriam marisque Baltici 

 Provincias nee non de Trilobites. Casani, 1825, p. 44. 



In this paper, Eichwald, evidently not understanding Brongniart's 

 genus Asaphus, describes the new genus Cryptonymus, and under it, 

 eight species. The first four are species of Asaphus and the last four 

 are species of Illaenidae. Asaphus had been described in 1822, but 

 Illaenus was not published till 1826, so that, if the four asaphids were 

 eliminated, it would really leave an illaenid as the type of Cryptony- 

 mus. The first of the illaenids described, Cryptonymus roscnhergi, is a 

 Bumastus, the second, Cr. wahlenbcrgi, third, Cr. rudolphii, and the 

 last, Cr. parkinsonii, are all species of Illaenus. To choose among 

 these a type for Cryptonymus would be to upset one of two well- 

 established names without any possible gain. Eichwald himself ac- 

 cepted Illaenus as the name of these species, and in 1840 transferred 

 his name Cryptonymus to Trilobites pimcfatus Wahlenberg, a pro- 

 ceeding which he afterward stoutly defended (1855). If a new generic 

 name is at any time necessary for any of the eight species described 

 by Eichwald, Cryptonymus is still available, but till such a con- 

 tingency arises, it seems best not to revive the name. 



Illaenus Dalman, 1826. 



Type, Asaphus crassicauda Wahlenberg. 



Om Palaeaderna eller de sa kallade Trilobiterna. Kgl. Vet. akad. 

 Handl., 1826, p. 248. 



