258 
PALEONTOLOGY OF NEW YORK. 
occurring in tlie Shaly limestone of the Lower Helderberg group, R. aqairadiata, 
Conrad, R. elliptica, Hall, R. mutahilis, 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. CumherlandicE, 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 
increased 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, Henssel^ria is very similar to Centro- 
NELLA. This similarity has been frequently noticed and has been brought out 
with especial emphasis by OEhlert in his demonstration of the brachial supports 
of Terebratula 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 
Renssel^ria, 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 re'spect 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 (Centronella) Guerangeri; Bull, de la Soc. d’Etudes Scientif. d’Angers, 1883, 
pp. 1-11, pis. i, ii. 
