Page:The Eurypterida of New York Volume 1.pdf/28

 excrescences. On the abdomen the scales are sometimes disk-shaped but more frequently v-shaped or crescentic, rising in posterior direction.

Nieszkowski [ 1859, p. 335] has suggested that these scales are the attachment places of muscles, a view which to us seems to explain most peculiarities of their structure and distribution. The following points seem to support Nieszkowski's assertion:

1 The scales are more distinctly outlined on the inside than on the outside of the test. In some cases they are well seen on the interior while faint or invisible on the outside, as in the specimens of.

2 On the abdominal segments they are distributed in distinct transverse zones or belts [see, pl. 31; Pterygotus, pl. 79] which correspond to the muscle bands that function in moving the segments, and they are entirely absent over the anterior and posterior doublures where no muscles could be placed; on the dorsal side they are arranged in longitudinal rows that follow the course of the intestinal canal and indicate the insertion of muscles with a suspensory function.

3 They are especially strong and distinct on the movable plates of the underside of the abdomen which carried the gill plates and were strongly shifted forward and back in breathing and swimming.

In the mature Limulus the crust is too thick to show any such muscle impressions on the outside, and moreover, the body has become separated into three solid fused regions (the cephalothorax, abdomen and telson) which only are movable upon each other. The muscles have thus become localized and fastened to strong internal processes or entapophyses. No such entapophyses have been found in the integument of the eurypterid abdomen, and there is in these structures evidence of the primitive condition of the musculature indicating a state of dissolution into many small muscular fasciculi. The thin-shelled young Limulus, less removed from the distinct abdominal segmentation of the embryonic stage, still exhibits a like distribution of the muscles and also shows from the outside such