258 PALEONTOLOGY OF NEW YORK. 



occurring in the Shaly limestone of tlie Lower Helderberg group, R. aquiradiata, 

 Conrad, R. elliptica, Hall, R. mutabilis, Hall, in all of which the form is tere- 

 bratuloid, the umbonal slopes very gradual, not full and squared, the beak of the 

 pedicle-valve prominent and suberect, exposing the undisturbed deltidial plates ; 

 while the muscular scars are but feebly developed. Similar characters are shown 

 in R. Cumberlandicz, of the Oriskany fauna at Cumberland, Maryland, where pre- 

 dominating species of this formation are associated with many Lower Helderberg 

 types. In R. Cayuga, sp. nov., of the later Oriskany of Ontario, the elongate- 

 oval form of the shell is maintained with close incurvature of the beaks and 

 inci'eased convexity of the brachial valve. The high-shouldered form and 

 linguate outline of R. ovoides, which it shares with Amphigenia, is also well 

 exemplified in R. Marylandica. 



In the structure of its brachidium, Renssel^eria is very similar to Centro- 

 NELLA. This similarity has been frequently noticed and has been brought out 

 with especial emphasis by ^hlert in his demonstration of the brachial supports 

 of Te.rebratula Guerangeri, de Verneuil,* though we can not follow this author 

 in his reference of such plicated shells to the genus Centronella. This genus 

 bears upon the surface of the broad anterior plate a median ridge which projects 

 a little at each extremity ; in C. Julia, A. Winchell, this ridge becomes a promi- 

 nent vertical plate. There is, however, in these shells no posterior and upward 

 projection of the ridge, as in Renssel^ria. In the earlier and smaller forms of 

 RensseljERIA, the hinge-plate is thin and perforated by an oval foramen which 

 lies below the apex of the umbo. It is also distinctly divided by diverging 

 ridges at whose extremities originate the crura. This structure is precisely that 

 of T. Guerangeri, and in respect to the brachidium throughout there is no material 

 difference except in the lesser development of the anterior plate in the latter. 



Rensselceria mutabilis, Hall, of the Lower Helderberg fauna, which is the 

 smallest known representative of the genus, frequently exhibits a primitive 

 condition of the brachidium in the imperfect coalescence of the lateral parts of 

 the anterior plate and the development of a median ridge upon its lower side. 



* Note sui- Terebratula (Ceiiti'iinella) Guerangeri ; Bull, de la Soc. d'Etudes Scientif. d'Angers, 1883, 

 pp. 1-11, pis. i, ii. 



