150 PALAEONTOLOGY OF KENTUCKY. 



Our shells are more transverse, and show a somewhat geniculated base like 

 Slrophodonta rhomboidalis, and besides, are considerably smaller ; but these 

 differences are only the results of local conditions. 



Formation and Locality. Found in the Niagara limestone in the quarries east of the city of 

 Louisville, Ky. 



Genus Strophomena. 



Strophomena, Rannesque. Manuel de Malac. of Blainville 1825. 

 Etymology: strophos, bent; mene, a crescent. 



The Genus Strophomena was established by Rannesque, and has for its type 

 the S. rugosa, which is identical with Strophomena rhomboidalis of Wahlen- 

 berg, a species which ranges from the Lower Silurian to the base of the Car- 

 boniferous system. This species has some little differences in shape and mark- 

 ings, according to the formations in which it appears. I have not been able 

 to see Rannesque' s original description ; but whatever this may be, it covers all 

 the shells which are now included in this genus and those of Hall's genus, 

 Strophodonta, which latter will certainly be discontinued. 



Strophomena rhomboidalis. WAHLENBEKG. 



Plate XVIII., figures 1, 2 and 3. 



Strophomena rhomboidalis, Wahlenberg. Acta. Soc. S. Upsaliensis, Vol. 8 1821. 

 Strophomena rhomboidalis, Hall. Pal. N. Y., Vol. 4, p. 761867. 

 See list of synonyms in last named report. 



Shell of more than medium size ; semi-elliptical or sub-quadrate ; varying 

 greatly in its proportions of length and breadth; hinge-line straight, and 

 equal to greatest width of shell ; cardinal extremities mostly rectangular, 

 sometimes salient. The valves are geniculated, and the proportions of the 

 flattened part of the disc and the recurved part of the shell are very variable, 

 insomuch that the geniculation is sometimes little more than one of the strong 

 concentric wrinkles. (Hall.) 



In the specimens of this species, found in our rocks, there is not so much 

 variation as stated in the foregoing description of Prof. Hall. Our shells 

 have almost all the form, as shown in the illustrations. They are sub-quadrate; 

 their lateral margins are parallel and almost straight, and the basal margin is 

 broadly curved. The geniculation is usually found in both valves, although 

 in some specimens the dorsal valve does not possess any, but is entirely plane. 

 The deflected portion of the lateral margins is triangular, with its apex at the 

 cardinal extremity ; it increases gradually towards the base, where it equals 



