ORDOVICIAN FOSSILS 



49 



the beak on the hinge line. In some of them grooves and ridges fan out from the beak; these 

 are called radiating ornamentation. They may be crossed by others that follow the curved 

 outline of the shell; these are called concentric ornamentation. We shall first take up the 

 species in which the concentric ornamentation is stronger and then those with stronger radi- 

 ating ornamentation. 



Two genera, Leptaena and Sowerbyella ( Plectambonites in older 

 publications) have stronger concentric than radial ornamentation. 

 Leptaena is easily distinguished by the downbending of the shell toward 

 the margin; it has strong concentric wrinkles near the beak and weak 

 ones on the downbent part of the shell. Fine radiating ornamentation 

 cuts these wrinkles. Leptaena richmondensis (fig. 59) is found in the 

 entire Richmond. In Sowerbyella rugosa (fig. 60) the shell is not bent 

 downward, the wrinkles are weaker and near the edge away from the 

 beak; it occurs in the Waynesville and Liberty formations of the Rich- 

 mond. 



Fig. 59 



In all our remaining species, the radiating ornamenta- 

 tion is stronger than the concentric. Some of them have 

 few, strong, radiating ridges and others many weak ones. 

 They may be divided into those with less than 40 such ridges 

 and those with more than 40. 





Fig. 60 



Fig. 61 



Two of our genera with less than 40 ridges have a 

 strong ridge, called a fold, down the center of one valve 

 and a corresponding groove, called a sinus, on the other. 

 In the genus Lepidocyclus ( Rhynchotrema in older pub- 

 lications) the shell is round and fat, with a pointed beak. 

 We have two common species, Lepidocyclus capax (fig. 

 61) of the whole Richmond, which is round and fat and 

 and has 3 ridges in the sinus and sharp concentric 



ridges crossing the radiating ones. L. dentatum, also found in the whole Richmond, is small- 

 er than L. capax , has only one ridge in the sinus, and lacks the sharp concentric ridges of 

 of that species. 



In the genus Platystrophia (fig. 62) also with less than 40 ridges and with a sinus and 

 fold, the shell is straight along the hinge and it has the appearance of two short bird wings. 

 There are many species in our Cincinnatian, each with a particular geologic range. No 

 attempt is made here to distinguish them; the figures in Bucher (1945) will identify the com- 

 moner ones. 



In Zygospira the sinus and fold are weak but distinct, the outline of the shell is 

 rather rounded; the shells are small, about l/4 inch or less wide. Z. modesta (fig. 

 63) is found throughout the Cincinnatian; it is small even for the genus, less than 1/4 

 inch wide, has a rounded outline and about 6 plications on each side of the shell. 



Fig. 63 



