THE EURYPTERIDA OF NEW YORK 33 
plate, termed the endocranium, plastron or entosternon,’ which in its turn 
forms the fulcrum for other important muscles. Owen has shown that 
the glabellar furrows, or rather the entapophyses or infoldings to which 
the furrows correspond, are the bases of the powerful levators of the preab- 
domen and also of the muscles which serve to steady the entosternon 
while the latter furnishes the fixed points for the flexors or depressors of 
the preabdomen and the important muscles that hold and move the coxal 
joints of the legs. 
Figure 6 Longitudinal section of Limulus 
liv, liver; pr, proventriculus; sé, stomach; Ht, heart; cp, cartilaginous plate, entosternon; 2, intes- 
tine; a, anus; br, brain. (From Packard) 
The glabellar furrows of Eurypterus thus correspond quite precisely 
‘in position and extension to those of Limulus and the appendages on the 
ventral side are entirely homologous and of like position and _ struc- 
ture; hence it is to be inferred that the same system of muscles existed in 
Eurypterus as in Limulus, that also Eurypterus may have possessed a 
cartilaginous entosternon and that the glabellar furrows served as bases 
for the levators of the preabdomen and the lateral levators of the ento- 
sternon. These furrows are also well developed in species of Stylonurus, 
as S. cestrotus. | | 
‘The importance of this internal skeleton to the muscular system of Limulus is 
fully described by Owen [Palaeontographical Society 1878. 32: 187]. 
