452 CARBONIFEROUS FORMATIONS AND KAITNAS OF COLORADO. 



Thus it is possihlo tluit Dentalium sp. and this form referred to D. suhlsmie may 

 prove to be the same. 



Locality and hoi'lzon. — San Juan region (stations ^219, 2238); upper portion of 

 the llei'inosa formation. Crested Butte district (station 2303); Maroon formation. 



Dentalium sp. 



At station 2287, in the southern portion of the Silverton quadrangle, occur rather 

 plentiful fragments of an organism which, though not altogether pi-oblematical, is 

 none the less interesting. 1 have little doubt that it is a Dentalium of an unusuallj' 

 large size. The largest fragment is 85 mm. long and not less than 20 mm. in 

 diameter. As the shell substance is in places at least 8 mm. thick, the complete 

 diameter of this portion of the shell was probably 25 or 30 mm. The fragment 

 shows but little taper, and the shell must have been, when entire, of almost unex- 

 ampled size and massiveness. The central portion, where exposed to view, is 

 unchambered, and the shell can not, therefore, be an orthoceratoid, unless the 

 fragment represents only the chamber of habitation. The microscoj)ic structure is 

 distinctlj^ molluscan and agrees especiallj' with Dentalium. In the Cisco division 

 of the Carboniferous, near Graham, Young County, Tex., an equally large Dentalium, 

 of a possibly undescribed species, is found. A single incomplete specimen of this 

 form measures 200 mm. in length, and other shorter pieces of larger size indicate 

 that it must have attained a length of 300 mm. and more. The diameter of these 

 large fragments is upward of 30 mm. and the shell is 4 or 5 mm thick. There is 

 no doubt about the affinities of the Texas form, whose existence relieves a reference 

 to Dentalium of the specimen from Colorado of most of its incredible features. 



Locality and horizon. — San Juan region (station 2287); middle portion of the 

 Hermosa formation. 



PLAGIOGLYPTA Filsbry, 1900. 



To Plagioglypta belong, probabl)% all the other American Carboniferous Dertta- 

 lia known up to the present time. Among these species two, and possiby three 

 groups are distinguishable, the annulistriata group, the meekiana group, and the 

 venvstagvowj}. 



Unique among oxxv Ca.vhomievo\i8 Dentalia in the strength of its annular mark- 

 ings stands Meek and Worthen's species, Plagioglypta annulistriata. It is too 

 marked a form to require comment here, and is the onlj^ representative of the annuli- 

 striata group, to which it lends its name. Much finer and less conspicuous are the 

 strife which traverse Plagioglypta meekiana., 3'^et still distinct enough to give 

 character to its sux"face. With PI. meekiana in the meekiana group can probably be 

 associated PI. primaria, PI. canna, PI. grandseva, and PI. ' granmllensis. As PI. 



