aia palj:ontology of new-york. 



Pig. 2 a, b. Dorsal and ventral views of the specimen, natural size. 



Fig. 2 c, d. Front and profile view of the same. 



Fig. 2 e,f. Dorsal and front views of a larger specimen, the lateral extremities of which 

 arc broken off. 



Geological position and locality. In shaly limestone of the age of the Lower 

 Helderberg limestones : Decatur county, Tennessee. 



Treiuatospira caiiiura. 



Plate XXVIII A. Fio. 1 a - h. 

 Mn/pa camitra : Palaeontology of New- York, 1850, Vol.ir, pa. 273, pi. 56, f. 3. 



Shell transversely elliptical or subrhomboidal, sometimes depressed sub- 

 globose (varying much in form according to age) : valves nearly equally 

 convex. Ventral valve having a narrow well-defined sinus, which is 

 usually marked by one or two plications smaller than those on the 

 lateral portions of the shell, and which become obsolete before reaching 

 the beak : beak elevated, small, acute, slightly incurved, and vertically 

 truncate by a small round or slightly oval perforation, the lower side 

 of which is formed by a well-defined deltidium. Dorsal valve with a 

 scarcely defined mesial fold marked by two smaller plications, which 

 usually become obsolete before reaching the beak : beak closely in- 

 curved beneath, the deltidium. 



SuRFAGE marked by about four, five, or six plications on either side of the 

 mesial fold and sinus, and concentrically crossed by imbricating lines 

 of growth, with the entire siu:face granulose or punctate. 



This species varies considerably in its form and proportions; being sometimes 

 depressed globose, with length and width about equal, while the prevailing forms 

 have the width nearly one-half greater than the length. In a single individual, 

 which does not appear to have been distorted, the width is twice as great as the 

 length. In all these variations, it preserves the characteristic surface markings. 

 Fig. 1 a. Dorsal view of a specimen of medium size. 



Fig. 1 6. Ventral view of the same, showing a single small plication in the mesial sinus. 

 Fig. 1 c. Front view of the same. 

 Fig. 1 d. Profile view of the same. ' 



