DESCRIPTIONS OF SPECIES. 469 



district (stations 22T3, 2375); upper portion of tlie Weber formation and in the 

 Robinson limestone. 



Bellekophon giganteus Wortiien? 



1884. Belleroplion glganfens. Worthen, Illinois State Mus. Nat. Hist., Bull. No. 2, p. 8. 



Lower Coal Measures: Monroe County, 111. 

 1890. Bellerophoh giganietis. Worthen, Geol. Surv. Illinois, Kept., vol. 8, p. 14.3, pi. 25, figs. 5, 5a. 



Lower Coal Measures: Monroe County, 111. 

 1S97. Belleroplion giganteus. Ulrich, Geol. Nat. Hist. Surv. Minnesota, Final Rept., vol. 3, pt. 2, p. 853. 

 1899. Bellerophon giganteus. Girty, U. S. Geol. Surv., Nineteenth Ann. Kept., pt. 3, p. 592. 



At station 2338 in the Rico formation occurs a Bellerophon of so large size that I 

 have tentatively referred it to B. gigcmfeus Worthen. This species was founded 

 upon an internal cast and no determinative characters are known except its shape 

 as such and its large size. The character last mentioned seems to have even less 

 specific significance among the Bellerophontacea than in other groups. Among what 

 seem to be mature examples well-known species show unusual variation in this 

 regard. For instance, in the case of B. perc/ri/u/tu.s, which is not often a large form 

 and where an axial diameter of 25 mm. would not, perhaps, be under the average, an 

 example now before me, apparenth* belonging to the same species, would have had, 

 if perfect, a dimension of over 100 mm. So that, as far as known, B. (/igant&us 

 might well be an unusual example of one of the well-known species. 



The specimens from Colorado are much broken, but some of them must have 

 attained a size much greater than ordinary. Another specimen in our collections 

 which, so far as mv material permits me to judge, is the same species as that from 

 Colorado is said to have been found at Alton, 111. , and in the Coal Measures. The 

 following notes are taken froui the Colorado material. Fully grown specimens 

 probalily had a diameter of at least 85 mm. measured in the plane of revolution. A 

 transverse section near the aperture i.s not a regular curve, but rather has the shape 

 of a (jothic arch, being somewhat pointed at the top with a raised slit band. The 

 surface is marked by rather strong transverse striae or sublamellose growth lines 

 which bend backward more or less strongly as they approach the band. The shell is 

 thick and massive, and the interior section i.s not far from a regular curve, the difl'er- 

 ence of contour between the inside and the out residing entirely in a median thick- 

 ening of the shell. In the earlier stages the outside curvature also seems to be 

 regular. . There is some evidence for believing that B. giganteus? togethei- with B. 

 cra-'txus and B. stevenmanus represent a single ontogenetic type. The chief, seemingl}^ 

 the only, points of diii'erence between B. giganteus? and what may be called typical 

 B. crassus are its large size and its keeled dorsum, but younger specimens of the former 

 do not present the carinate condition and are of course of smaller dimensions. What 

 I have reason to believe are young specimens of B. crassus are about the size and 



