132 NEW YORK STATE MUSEUM 
The main stem of the family Eurypteridae is formed by the genus 
Eurypterus, whose earliest form seems to appear in the Lower Siluric 
(Frankfort shale) and it persists into the Permic without materially chang- 
ing. Corresponding to the vigor shown by its longevity is the great pre- 
ponderance of its individuals and the number of species it has produced. 
Among the latter very different stages of progress are found. Some have 
clearly progressed but httle, as shown by their terete, undifferentiated 
abdomen, short blunt telson and short swimming legs. Such forms are 
E. maria Clarke, E. pygmaeus Woodward and E. minor 
Laurie. The form and small size of all these suggest an immature con- 
dition or an arrested development. This is emphatically true of the peculiar 
type for which Laurie [1899, p. 588] has proposed the genus Bembycosoma 
which has a semicircular carapace and a short, conical body, stout, short 
telson and apparently but nine abdominal segments. This form, repre- 
sented by asingle species, resembles nothing more than our eurypterid 
larvae from Otisville and it is very possible that these and some other 
primitive-looking small species of Eurypterus are cases analogous to ‘those 
occurring among the recent crustaceans where development has been 
arrested by parasites which prevent molting and result in dwarfed adults 
retaining larval characters. | 
One species of Eurypterus, viz, E. kokomoensis [pl. 25, 
fig. 1] is of special phylogenetic interest as pointing the way which has 
led from Eurypterus to Dolichopterus, Drepanopterus and Stylonurus. 
These genera form a group by themselves, well defined by a number of 
peculiar characters, the most important of which are the great lengthening 
of the legs, especially of the last pair; the slender body without distinct 
differentiation of preabdomen and postabdomen and the slender, styliform 
telson. Connected with the enlargement of the cephalothoracic appendages 
is that of the carapace which is relatively longer than in all other euryp- 
terids, shows a distinct tendency to become squarish, and develops a broad 
border. Likewise the greater development of the legs affects the metas- 
- toma which in all these genera is very long and subrectangular with deep 
