68 



visible, exhibiting concentric stria*. The 'whole Sur- 

 face of the animal has a jet black polish. Length 

 from 6 to 12 inches. 



The original of our cast is in the cabinet of J. P. 

 Wetherill, and was found near Cincinnati, Ohio. It is 

 of a yellowish colour, and occurs in argillaceous slate. 

 Specimens are common in most cabinets of Ameri- 

 can fossils. The Lyceum in New York, possesses a 

 fragment of an individual of this species, which must 

 have been at least 17 inches long. Our cast, No. 22, 

 is from the gigantic tail in the cabinet of P. A. Brown, 

 Esq. Mr. Stokes describes the I. gigas as a new 

 species under the name of Asaphus Platycephalus, in 

 Geolog. Trans, vol. i. N. Scries. His specimen was 

 found in the limestone of. St. Joseph's, Canada. 



ISOTELUS PLANUS.* Dekay. Cast No. 23. 



Head more rounded than the preceding, and less 

 elevated. Tail flat, rounded. Total length two inch- 

 es and one-tenth. Breadth one inch and one-tenth. 

 Length of the head, six-tenths of the abdomen, 

 eight-tenths, and of the tail seven-tenths. 



Dr. Dekay is of opinion that this species may pos- 

 sibly prove to be the young of the preceding. The 

 relative proportions of its buckler and tail vary 



* The general usage of naturalists is to prefix a short Latin 

 caption to the species which they discover but as some authors 

 do not follow this fashion, we are satisfied to suffer their descrip- 

 tions to stand without it. We believe, indeed, that the time is 

 not very distant, when every author will be expected to publish 

 his discoveries in his vernacular tongue. 



