1888] MARYLAND ACADEMY OF SCIENCES. 27 



Rogers, are occasionally met with in the shell-rock, but, as no 

 perfect examples have yet been secured, a thorough comparison 

 of the two forms has not been possible. With regard to 

 the former, it may be said that more than one hundred speci- 

 mens of the casts, chiefly from the vicinity of Mattawoman 

 Creek, have been examined by the writer, and they have shown 

 quite a wide range of variation in form and proportions. Some 

 very large specimens were taken from the marl stratum in the 

 ravine back of Fort Washington, which were distorted so much 

 that the beak of one side was bent far out of line from its 

 fellow of the opposite side, while the curve of one valve was 

 flattened, and of the other was expanded so as to appear abnor- 

 mally convex. In certain other specimens much variation 

 also was observed in the width of the gap between the beaks, 

 while the beaks themselves showed marked differences in length 

 and curvature. 



Probably the most variable shell yet taken from our 

 Eocene beds is the Ostrea compressirostra Say. Hundreds of 

 specimens of this oyster have passed through my hands, and 

 I have examined large numbers of them of all sizes, ranging 

 from a diameter of less thau an inch up to that of eight inches. 

 Among the medium-sized specimens we find those with acutely 

 sinuated scales of growth on the lower valve, described as 0. 

 sinuosa by Prof. W. B. Rogers, while in the extremely large 

 and thick specimens the scales are wide, feebly or hardly sinu- 

 ated, and the wrinkles of the same valve nearly obliterated, as 

 in the 0. beUovacina, or without wrinkles as in the 0. gigas of 

 England. 



Scarcely more than two dozen species of fossil shells be- 

 longing to this formation have thus far been discovered within 

 the strata of the State of Maryland. Most of these, however, 

 are characteristic and conspicuous forms, generally bearing 

 only a remote resemblance to the species which preceded them 

 in the Cretaceous marls. Thus, in the genus Ostrea, which 

 belongs to more than one of the horizons of both formations, 

 the oblong or elongated simpler forms of the Mesozoic have 

 given place to the broad, wrinkled, and more complicated ones 

 of the Cenozoic. Here, also, as in New Jersey, we no longer 



