Page:The Eurypterida of New York Volume 1.pdf/162

 and     from the Frankfort shales complete the list of North American representatives of the genus.

In Europe a considerable number of species were described by Salter and Woodward but the principal advance in the knowledge of the genus and its structural details has been made by F. Schmidt [ 1883] and Holm [ 1898]. Both of these authors described the  from Oesel, which had already served as the subject of Nieszkowski's investigations and was at that time identified with   DeKay. Schmidt corrected and supplemented in many ways Hall's and Nieszkowski's descriptions of the endognathites, demonstrated the presence of five separate sternites (Hall knew but one, the operculum, and Nieszkowski assumed six) and showed that they are open on the ventral side. He further pointed out differences in the appendages of the first and second sternites in individuals otherwise alike and attributed these to sexual differences. In the dolomite marls of Oesel these merostomes are not preserved as in the waterlimes of New York where the integument is fully carbonized. By most skilful manipulation Holm succeeded in removing from the matrix the integumetal film of  and was thus enabled to study these parts by transmitted light, and to describe the structure of the integument with a completeness that could be equaled only from the living organisms themselves. We shall not here enter upon a detailed review of the results of his investigations, but shall point out the more important of the determinations, since there is no reason to doubt that entirely similar structures existed in the closely related species  and. Indeed, our material verifies several of his detailed discoveries referred to below.

The absence of facets in the compound eyes was conclusively shown by transmitted light. On the underside of the cephalothorax the presence