valve. Area of the ventral valve from one to three lines in width, transversely striated for 

 from one-third to one half of the distance from the beak to tha cardinal angles. Dorsal area 

 narrow and linear. 



" Surface marked by angular, sub-equal, interrupted or rugose striae, which are often 

 made to appear more uniform by the partial exfoliation of the shell. The striae are bifur- 

 cated two or three times before reaching the margin, and they are sometimes increased by 

 intercalation. As usually preserved, they rise at intervals into sharp ridges or elongated 

 nodes ; and in well-preserved specimens these are produced into short spines. When the shell 

 is exfoliated, the surface between the striae is marked by minute pores or tubular openings, 

 which communicate with pustulose elevations on the interior surface of the shell. In the en- 

 tire cast, these pores mark the surface with considerable regularity, varying in size, and uni- 

 formly larger below the middle, where the valve is more abruptly reflected 



" The muscular impressions of the ventral valve are very large and broad, and the mar- 

 gins distinctly limited by a curving elevated border. It is divided in the middle by a more 

 or less developed septum, which terminates in a prominent callosity beneath the place of the 

 foramen : this callosity gives a.bilobate character to the cast of the rostral cavity. The 

 place of the occlusor muscles is strongly marked, and that of the divaricator muscles distinctly 

 bilobed. The muscular imprints are about two- thirds as long as wide, and the width is often 

 nearly equal to half the width of the shell. The muscular impressions of the dorsal valve 

 are narrow and elongate, and are separated by a rounded median ridge, which divides above 

 and continues in two diverging processes, the full extent of which is not shown in any of our 

 specimens," (Hall, Pal. N. Y., Vol. IV., p. 94). 



Professor Hall places S. ampin, in his genus Strophodonta, and states that the place of a fora- 

 men is taken by a smooth triangular space with a narrow callosity along its centre. According 

 to Mr. Billings, however, there is a small linear closed foramen, one-third of an line in width. 

 It seems very doubtful if there is any difference of specific value between S. ampla and the 

 more ancient Lower Helderberg species, S. punctulifera ( Conrad), . Headleyana (Hall), and ib. 

 cavurnbona (Hall ). Mr. Billings points out that S. ampla and S. punctulifera are substantially 

 identical, the Corniferous specimens, however, being on an average larger than those figured 

 by Hall from the Lower Heidelberg. I have, however, collected examples of S. punctulifera 

 the dimensions of which are fully equal to those of the Devonian form. It would probably be 

 wise, therefore, to substitute S. punctulifera for <S'. ampla, retaining the names HeatHa/ana 

 and cavumbona in the meanwhile as designations of varietal or sub-specific value, thus carrying 

 out Professor Hall's own view that a more extensive series if specimens will show that all these 

 forms belong to " one species presenting variations of size, degree of convexity, width of area, 

 and difference of surface-markings due to the influence of the sediment and other surrounding 

 conditions which affect the development of animal life." (For descriptions and figures of S. 

 Headleyana, S. cavumbona, and S. punctulifera, see Palseontol >gy of New York, Vol. Ill, 

 p. 185, PI XX., Figs. 13 ; p. 187, PI. XXI, Fi-s. 13, and' p. 188, Plate XXI, Fig. 4, 

 and PI. XXIII, Figs. 47. 



Strophomena ampla may as a rule be recognized by its peculiar form and the characteris- 

 tic ornamentation of the surface. The commonest appearance is when the shell is partially ex- 

 foliated, and the puncta between the striae are visible. Spines cannot usually be detected, 

 but Mr. Billings has figured a specimen in which the whole surface is adorned with short 

 spines. 



Locality and Formation. Corniferous Limestone of Port Colborne. 



81. STROPHOMENA NACREA (Hall). 



Stropliomena (Strophodonta) nacrea (Hall), Tenth Report on the State Cabinet, p. 144, 

 Strophodonta lepida (Hall), Geol. Report, Iowa, Vol. I., Part ii., p. 493, PI. III., figs. 3, 



a, b, c. 



Strophomena lepida (Billings), Journ. Can. Institute, New Series, Vol. VI. p. 344. 

 Strophodonta nacrea (Hall/, Pal. N. Y., Vol. IV., p. 104, PI. XVIII., figs. 1, a, b. 



Shell small, from four to nine lines wide, and from three to six lines long, semi-circular, 

 or sub-quadrate, the hinge-line crenulated and as wide as, or wider than, the shell below. 

 Front margin rounded, cardinal angles rounded or extended and angular. Ventral valve 



