42 NEW YORK STATE MUSEUM 



valve ii. From this point outward the ribs irregularly dichotomize each 

 into two or sometimes three, fold and sinus being affected like the rest of 

 the surface. 



The shell is transverse with straight hinge and without cardinal areas. 

 The ventral beak is abruptly perforate and the shell substance punctate. 



Horizon. No. 9. 



Atrypa reticularis Linne 



Very abundant and without variation from the Helderbergian type. 

 Horizon. Nos. 8, 10, 11. 



Stropheodonta 



It is quite clear that the Stropheodontas of the Helderbergian fauna 

 find their origin in the stock of that variable, unstable and shifty shell whose 

 protean expressions have been usually embraced under the name Stro- 

 pheodonta varistriata Conrad, occurring in the Tentaculite or 

 Manlius limestone of New York and passing upward into the Coeymans 

 limestone of the Helderbergian. We have already had occasion to refer 

 to this species. It is first, a Brachyprion in the sense that its row of 

 cardinal denticulations does not extend far from the delthyrium ; secondly, 

 its surface may be either coarsely ribbed, finely ribbed and fasciculate or, 

 with the last condition, puckered or undulated. All these expressions have 

 been shown in accounts of the shell \see Palaeontology of New York, v. 8, 

 pt 2, pi. 13, fig. 6- 1 6]. These differences are thus accounted for: the 

 coarse plication is the perdurance to maturity of a primitive condition not 

 modified in later life ; the condition of finer plication results from an 

 acceleration of intercalation of plications ; fasciculation follows, or in senile 

 instances may precede this multiplicate condition. Undulation of inter- 

 spaces follows the initiation of the fasciculate stage. The relative time 

 of appearance of these features will depend wholly on the degree of 

 acceleration or retardation in ontogeny. 



The specific name S. varistriata now stands for a series of small 

 shells with these variable expressions. Occasionally these are doubtless 

 adults, never attaining large growth as in the Manlius limestone. In the 

 faunas of next later date, however, all young forms of the regular or normal 

 Stropheodontas are S. varistriata of one type or another, or to put 

 the case conversely, those Stropheodontas can be traced by the surface 

 markings on the adult shell, back from their mature stages whatever these 

 may be, to one or another of the primitive expressions of S. vari- 

 striata. But in any such fauna, when all specific identities of mature 

 forms have been eliminated there remain behind series of younger shells of 

 variously progressed conditions which are not always readily assigned to 

 their proper so called species. We have observed this in the Helderberg, 



