. Annual report of the regents of the university of the state of New York on the condition of the State Cabinet of Natural History and the historical and antiquarian collection annexed thereto. Science. 268 TWENTIETH REPORT ON THE STATE CABINET. and the spiral coils are much more lax. These features, however, might not he of generic importance. Fig. Atrypa reticularis : The interior of the upper part of the dorsal valve, showing the connecting loop of the spires. In external characters, the Zygospira and its congeners differ from Atrypa in the depressed dorsal valve, with a distinct fold e
. Annual report of the regents of the university of the state of New York on the condition of the State Cabinet of Natural History and the historical and antiquarian collection annexed thereto. Science. 268 TWENTIETH REPORT ON THE STATE CABINET. and the spiral coils are much more lax. These features, however, might not he of generic importance. Fig. Atrypa reticularis : The interior of the upper part of the dorsal valve, showing the connecting loop of the spires. In external characters, the Zygospira and its congeners differ from Atrypa in the depressed dorsal valve, with a distinct fold embracing two or more simple plications ; while the ventral valve bears a median fold which is marked by strong simple plications, or divided only close to their origin at the apex of the shell. The plications are all simple, showing no tendency to bifurcate below their origin, which is at or near the apex, and in this respect resembling Rhynchonella. So far as we are able to determine, the teeth-sockets are smooth, and not crenulate as in Atrypa. In the characteristic species of the Genus Atrypa, the valves are convex, without distinct median sinus or fold, except towards the front of the shell. In the young shells the dorsal valve is often very depressed convex, and the beak of the ventral valve extended and perforate. The plications or costse are rounded and frequently bifurcated, with numerous concentric lamellose striae ; differing in this respect very essentially from the simple angular plications of Zygospira. These are the most obvious differences between the genera, and are, I think, sufficient to distinguish them from each other. I believe, moreover, that there are important differences otherwise, and that a critical study of the fossils demands that these forms be generically distinguished.* * The forms distinguished asC(ELOSPiRA are concavo-convex shells, with bifurcating and lamellose plications, and without distinctive mesial fold or Please note that these
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