Page:The Eurypterida of New York Volume 1.pdf/122

 6 The abdomen lacks distinct differentiation into pre- and postabdomen. This condition is made clear by the nearly uniform length of all segments. In width there prevails considerable difference, most early growth stages appearing to contract more rapidly than the mature individuals, while in Hughmilleria the body is distinctly more slender [ pl. 64, fig. 1]. In,   and especially in  , as well as in   there is evidence of a relatively broader and shorter preabdomen, although the more compact appearance of the preabdomen may be partly due to casual contraction after death.

7 The number of segments is less in the nepionic stage than in the following stages. In some nepionic individuals there are not more than six segments or half the number normal to the adult [see, pl. 51, fig. 1]. On account of the smallness of the earliest growth stages and the lack of differentiation of the segments we have not been able to satisfy ourselves as to whether the preabdomen or the postabdomen is first completed. The larva of  [ pl. 64, fig. 1] where a distinct preabdomen with four segments and a postabdomen of six segments are noticeable, and some other specimens, suggest that the latter was first completed.

8 The telson spine may have been less developed than in adults; at least such a condition is indicated by several specimens. Unfortunately and singularly the telson is nearly always missing in young specimens. In the embryo of Limulus there is no tail spine and in the larval stage the telson is small.

It is now interesting to inquire which of these ontogenetic characters are purely larval acquisitions, and which are palingenetic, or of phylogenetic significance as representing the recapitulation of ancestral characters. Comparison with the larval stages of Limulus has been of much assistance in discerning between these coenogenetic and palingenetic structures.

The larger size of the carapace we consider as a merely larval character, for the following reasons: This larger size is principally due to the