CRANIA 43 



Family CRANIID^ 

 Genus CRANIA Retzius 



Description. Shell subcircular in outline, usually more or less unsym- 

 tnetrical in its growth, inequivalvate. Pedicle valve without perforation 

 for the passage of a fleshy pedicle, attached by its apex or by its entire 

 external surface to some external object. Brachial valve depressed- 

 subconical, with a subcentral, erect or posteriorly directed apex. The 

 interior of each valve is marked by two pairs of large adductor muscular 

 scars, the posterior pair being close to the margin and widely separated, 

 the anterior pair being much closer together and subcentrally located. 

 The external surface of the brachial valve marked by more or less irregu- 

 lar concentric lines of growth, by regular radiating costas, or by fine 

 setse or pustules. 



Remarks. The more regular examples of the brachial valves of Crania 

 may be distinguished from members of the genus Orbiculoidea by the 

 more nearly central position of the apex and its more erect position. 

 These valves of the two genera also may be commonly differentiated by 

 reason of the less regular manner of growth in the genus Crania. 



It seems hardly justifiable to include in one generic group all the 

 species which are here referred to the genus Crania. Such species as 

 Crania roivleyi having the shell marked by conspicuous radiating cosfee 

 can hardly be congeneric with such species as Crania, reposita, for in- 

 stance, which have a fundamentally different type of surface markings, 

 and perhaps the species marked by the fine setae or pustules should be 

 made to constitute a third genus. These three types of surface markings 

 persist throughout the Paleozoic faunas where these shells are pres- 

 ent, and so far as the writer is aware, there is never any question as to the 

 differentiation of the groups. No generic names are proposed for these 

 distinct groups of species at this time, however, it being deemed best to 

 defer such action until a larger number of species from all the different 

 geological time periods are more critically studied. 



CRANIA MISSOURIENSIS Weller 

 Plate I, Fig. 30 



1909. Crania missouriensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 292, 

 pi. 12, fig. 1. 



Description. Shell rather large for this genus, subcircular in outline. 

 The dimensions of the type specimen are : length 17 mm., .width 17 mm. 



Pedicle valve attached to some external object, in the case of the type 

 to the interior of the shell of a Productus, its characters not observed. 



Brachial valve depressed, with the apex rather obscure and situated ex- 

 centrically about one-third the length of the shell from the anterior mar- 



