BRACHIOPODA. 357 



IMnobolus |V) parvus.] 



Shell substance originally not very thick, apparently smooth and now replaced 

 by a coarsely crystallized calcite, a feature common to many species of the Trimer- 

 ellidae. 



Of this species we have but a single specimen from Minnesota, found by Mr, E. 

 0. Ulrich, which is in an excellent state of preservation. The interior characters 

 are shown in greater detail than appears to have been the case in the material from 

 Wisconsin upon which Prof. Whitfield based the species, otherwise the example 

 agrees with his description and illustrations. In size and form it is nearer to 

 Dinobolus schmidti Davidson and King,* from the Lyckholmer Schicht at Kirna> 

 Esthonia, a horizon nearly equivalent to the Trenton limestone of New York, 

 than to Obolellina canadensis and 0. magnificus Billings** from the Black River group 

 of Canada. The latter, of which only the exterior is known, are also much larger 

 species than D. ? parvus. 



The interior of D. ? parvus, as revealed in the Minnesota specimen, is highly 

 instructive, since it appears to have characters both of the Obolidoe and Trimerellidcv. 

 The structure of the cardinal region is similar to that of Obolella, while the platform 

 and its muscular scars are as in Dinobolus. The crescent so characteristic of Din- 

 obolus is, however, absent in D. parvus. The diverging elevations on each side of 

 the pedicle muscle scar in Oboklla crassa, (the cardinal scars), are also present in 

 D. ? parvus and occupy a position equivalent to the crescent of Dinobolus. It may 

 be that here is indicated the line of development of the crescent from the cardinal 

 scar of Obolella. The combination of these characters in D. ? parvus seems to require 

 its separation from Dinobolus, but until the interior is known of the species referred 

 by Billings to Obolellina it is better to leave it provisionally in the former genus. 

 Lower Silurian species of Dinobolus are also known from Esthonia (D. schmidti) and 

 Brittany (D. brimonti Rouaultf), but these have well-developed crescents. 



Formation and locality. Middle Galena, Wykoff. 



Collector. E. O. Ulrich. Type in the collection of C. Schuchert. 



"Quart. Jour. Gtiol. Soc. London, vol. xxx, p. 104, pi. xix, llgs. 5, ii, 1874. 

 **Canad. Nat. Geol. vol. ill, p. 441. 1858; vol. vl, a. ser., p. 329, 1872. 

 tSee Davidson's paper In the Geol. Mast., vol. vli, dpcade li. p. :i40. issn. 



