26 NEW YORK STATE MUSEUM 
like 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 Nieszkowski1’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 
KEurypterus pittsfordensis. 
2 On the abdominal segments they are distributed in distinct trans- 
verse zones or belts [see Eusarcus scorpionis, pl. 31; Ptery- 
gotus, pl. 79] which correspond to the muscle bands that function in 
moving the segments, and they are entirely absent over the anterior and 
pesterior 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 con- 
dition 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 
