88 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



producing a frayed appearance in that portion of the posterior margin." 

 From G. varians and C. brevis it is distinguished by certain details of 

 its ornamentation. In particular, its anterior margin is described as 

 bearing a " prominent, eccentric marginal ridge, from which frequent 

 bifurcations are sent off on either side, each offshoot being more atten- 

 uated and curved posteriorly on approaching the posterior margin, form- 

 ing throughout closely approximated, rounded ridges, of which there 

 are about fifty, counting along the inferior margin, and less than half 

 that number two-thirds the distance to the apex." 



There can be no hesitation in referring to this species the original of 

 Plate 5, Fig. 1, which is from the same horizon, and belongs to the 

 United States National Museum (Cat. No. 4845). This spine displays 

 the general outline and ornamentation very satisfactorily, notwithstand- 

 ing it has been much laterally compressed by mechanical agencies. It 

 has a total length of 15 cm., and maximum breadth of 3 cm. A sharp 

 and prominent marginal ridge extends for the entire length of the 

 anterior border, and forty or more longitudinal costse terminate along 

 the very oblique line of insertion. The only noteworthy particular in 

 which it differs from the type relates to the ornamentation, which is of 

 the decussated instead of the tuberculose pattern. Isolated and well- 

 marked tubercles predominate in the type, although it is stated that 

 along the anterior margin " they pi-esent the appearance of closely 

 approximated decussations, apparently the result of abrasion, the 

 entire crest of the ridges [costse] being reduced to a smooth polished 

 surface." But in the example before us distinct tubercles are almost 

 entirely wanting, and the costse are crossed by exceedingly numerous 

 transverse ridges, the crests of which were apparently sharp and smooth. 



That this difference in ornamental details is of minor importance is 

 proved by the conditions existing in C varians, where tubercles of 

 diverse shape are variously disposed in different parts of the spine. 

 They frequently surmount the costse in double or even triple rows, and 

 the " pairs of nearly circular tubercles often coalesce, forming a single 

 transverse tubercle, which latter is the prevailing, if not persistent form 

 in the posterior costte." ^ A similar fusion of adjacent tubercles to form 

 a single transverse ridge has already been noticed in C. longinodosus 

 (v. supra, p. 80), hence it is not surprising that this process should be 

 observed in a varying degree amongst different species, being sometimes 

 limited to particular spines of the same species, and sometimes to differ- 

 ent portions of a single spine. 



Formation and Locality. — Kinderhook Group ; Iowa and Illinois. 



1 Zoo cJi. (1875), p. 422. 



