RHYNCHOPORA 227 



Genus RHYNCHOPORA King 



Description. Shell rhyncholliform, below medium size, usually pentan- 

 gularly ovate in outline, the mesial fold and sinus well developed in the 

 anterior half of the shell, both valves marked by simple, subangular or 

 rounded plications which are sometimes longitudinally grooved towards 

 the front. Internally the dental lamellae are well developed in the pedicle 

 valve. Brachial valve with a strong median septum in the rostral por- 

 tion, which is divided internally to form the walls of a crural cavity ; this 

 is covered over on its cardinal side by the undivided hinge-plate, the 

 hinge-plate continuing anteriorly beyond the anterior termination of the 

 crural cavity; the crura formed by the anterior extension of that part 

 of the hinge-plate opposite the lateral walls of the crural cavity. Shell 

 structure punctate. 



Remarks. No opportunity has been afforded the writer to make a 

 study of the internal characters of R. geinitziana, the genotype of Rhyn- 

 chopora, so there is a possibility of a wrong interpretation of the genus 

 in this place. Up to the present time the one essential generic character 

 which has been recognized is the punctate structure of the shell, but in 

 R. pustulosa, the only American species recognized by Hall and Clarke 1 

 and by Schuchert; 2 the undivided hinge-plate and the complete closure 

 of the crural cavity except anteriorly, are good differential characters. 

 All other species included in the genus in this place, except one, agree in 

 the character of the hinge-plate and crural cavity with R. pustulosa. 

 The puncate structure of the shell is conspicuously developed in only 

 one of the species here recorded, R. beecheri, which has been observed only 

 in the condition of internal casts and impressions of the exterior, no 

 serial sections of the rostral portion of the shell have been made, but the 

 hinge-plate and crural cavity are apparently as in R. pustulosa. The 

 Pennsylvania!! species R. illinoisensis is another one which is conspicuously 

 punctate, but its internal characters have not been worked out. In R. 

 pustulosa the punctate structure of the shell is so obscure as to almost 

 lead one to believe, at times, that it is absent, and in one or two of the 

 species here included in the genus no punctate structure has yet been 

 observed. Another character which differentiates most of these shells 

 from other Mississippian rhynchonelloids is the longitudinal grooving of 

 the flattened plications towards the front, especially those of the fold 

 and sinus, this condition being altogether similar to that in Wilsonia 

 and some other older rhynchonelloids. This character is not shown upon 

 any of the illustrations of R. geinitziana which have been available for 

 examination, and it is not present in R. hambiirgensis described in this 

 work. On the whole, R. hamburgensis seems to correspond most closely 



1 Pal. N. Y., vol. 8, pt. 2, p. 210. (1894.) 

 2Synop. Am. Foss. Brach., p. 366. (1897.) 



