THE EVIDENCE OF THE THYROID GLAND IgI 
operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, 
ete., belonged to the type of Thelyphonus, rather than to that of 
Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the 
ventral aspect of Eurypterus, and by the side of it a picture of the 
isolated operculum. Schmidt considers that there were five branchix- 
bearing segments constituting the mesosoma, the foremost of which 
formed the operculum. Such operculum is often found isolated, and 
is clearly composed of two lateral 
appendages fused together in the 
middle line, of such a nature as to 
form a median elongated tongue, 
which lies between and separates 
the first three pairs of branchial 
segments. This median tongue, 
together with the anterior and 
median portion of the operculum, 
concealed, in all probability, accord- 
ing to Schmidt, the terminal parts 
of the genital organs, just as the 
median part of the operculum in 
Phrynus and Thelyphonus conceals 
the complicated terminal portions 
of the genital organs. The posterior 
part of the operculum, like that of 
Phrynus and Thelyphonus, carried 
Fic, 78.—Hurypierus. 
The segments and appendages on the 
if i right are numbered in correspon- 
the first pair of branchize,so Schmidt = gence with the cranial system of 
thinks from the evidence of markings _lateral nerve-roots as found in verte- 
: brates. M., metastoma., The sur- 
scser nna: sabermmeney face ornamentation is represented 
Apparently an opercular ap- on the first segment posterior to the 
pendage of this kind is in reality branchial segments. The opercular 
the result of a fusion of the genital  *PPendage is marked out by dots. 
operculum with the first branchial appendage in forms such as the 
scorpion; for, in order that the tergal plates may correspond in 
number with the sternal in Eurypterus, etc., it is necessary to 
consider that the operculum is composed of two sternites joined 
together. Similarly in Thelyphonus, Phrynus, etc., this numerical 
correspondence is only observed if the operculum is looked upon 
as double. 
A restoration of the mesosomatic region of Eurypterus, viewed 
