Page:Walcott Cambrian Geology and Paleontology II.djvu/45

 Relations to pre-Camhrian Merostomes.—The fragmentary remains that were described under the name of Beltina danai were referred to the Merostomata, and the genus was considered to be more or less closely related to Eurypterus and Pterygotus. All the original specimens are flattened in a calcareous shale and none of them show definite surface markings. In a collection made by Prof. Stuart Weller in the Altyn limestone in the valley of Swift Current Creek, Montana, the specimens are embedded in a very fine calcareoarenaceous matrix, and many of them show the convexity, and some, the original surface markings. One of these (illustrated on pl. 7, fig. 4), an abdominal segment, shows the convexity and general form of the segment, and the surface is more or less roughened by what appear to be depressed tubercles.

Specimens collected from about the same horizon to the north in British Columbia, and embedded in a siliceous matrix, are more flattened than those from the Altyn limestone, but they show certain definite surface characters. Two of the specimens are illustrated on pl. 7. Fig. 2 is a portion of a cephalo-thorax, with irregular transverse ridges near the posterior margin and depressed tubercles over other portions of the surface. An abdominal segment (fig. 3) shows depressed tubercles not unlike those shown by the segment from the Altyn limestone illustrated by fig. 4.

The relations of this very ancient form to the Middle Cambrian Merostomes described in this paper are very uncertain owing to the fragmentary character of all the specimens of Beltina yet discovered. Most of these fragments are quite similar to fragments of Sidneyia inexpectans where the latter is broken up and flattened in the shale, but, as a whole, the form of all the parts of Beltina thus far recognized indicates a closer relationship to the Eurypterida than to the Sidneyidae.

Body elongate, with a thin epidermal skeleton either smooth or ornamented by lines or ridges. Cephalo-thorax with lateral or marginal eyes, on the ventral side with five pairs of movable appendages; mouth posterior to a large epistoma.