Page:AmJourSci 4 39 233 551-554.djvu/3

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553
E. W. Shuler—New Ordovician Eurypterid.

tal one being the larger. The alveolar processes in all the segments have a distinct serial arrangement along a median line. The pronounced flattening of the bristles or spines gave a much enlarged paddle-like surface to the whole endognathite.

The anterior margins of the segments show a smooth arcuate curve without dentition. The joints show the expected thickening around the articulating surfaces.

The second appendage, fig. 3, also shows three segments. These are shorter and more robust than the joints of the appendage just described. They do not show spines, though this may be due to the conditions of preservation. The fragment is probably a part of the first endognathite. The measurements, beginning with the proximal joint, are as follows: length, 8mm, 7mm, 7mm; the corresponding widths are: 4mm, 2·5mm, 2mm.

A part of the telson spine with the impression of the last two abdominal segments is shown in fig. 3. The posterior segment has a length of 16mm; the second has a length, estimated from the curvature of the mould, of 19mm. A number of measurements of Stylonurus and other Eurypterida showed a rather constant ratio between the length of these two segments and the total length of the individual. This ratio was approximately one to eight. While no special stress is laid on such a ratio, it is interesting that this would indicate that the complete specimen was 280mm long (about eleven inches).

Fig. 6 represents what appears to be a segment of one of the walking or balancing legs, typical of Stylonurus. It has a length of 30mm, but this probably does not represent the total length of the segment. The width at the proximal end is 4mm; at the distal end, 2·5mm. Fig. 5 shows a fragment of an unidentified limb.

The fragment of the carapace shows the mould of one of the compound eyes, two depressions questionably identified as ocelli, and the impression of a rough tubercular surface. The supposed ocelli lie well back behind the compound eye and the general appearance is more that of Eusarcus than Stylonurus. The state of preservation is, however, such as to preclude a definite identification or exact description. Fig. 2 shows an enlarged view (× 2) of the tubercular moulds.

This merostome has been provisionally referred to Ctenopterus, a sub-genus of Stylonurus, on account of the long flattened segments of the endognathite; the lack of serrations on the segments such as is found in Eurypterus; the presence of the long joint belonging probably to the walking leg or balancer, characteristic of Stylonurus; and the long broadened spines or bristles which are especially characteristic of the sub-genus Ctenopterus. It has been given the specific name alveo-