PTERYGOTUS BILOBUS. 67 
But there is yet another specimen, which seems to complete the evidence of the 
position of these branchial plates in Pferygotus. 
I allude to a specimen described in the ‘ Memoirs of the Geol. Survey,’ Monograph I, 
p: 49, under the name of Pé. perornatus, var. plicatissimus. One of the body-rings is 
there figured, Joc. cit.; and the anterior portion of the specimen is represented in pl. 1, 
fig. 16, of that work. We have figured the whole of the same specimen of the natural size 
in Pl. XI, fig. 2 a, and the anterior portion enlarged twice the natural size in fig. 2 6. 
Mr. Salter writes as follows :—“ The carapace’ is much compressed longitudinally, but 
the true form would probably be a full semioval; the position of the eyes is obscurely 
marked. ‘lhe surface is covered with semicircular plice,’ the curves of which open back- 
wards (contrary to the usual position over the rest of the body). In front is seen the 
impression of the large ovate median lobe of the epistoma,3 shaped as usual, and behind 
are two radiated muscular impressions‘ placed low down on the carapace and towards 
the median line, which impressions are probably the attachments of the great swim- 
ming-feet.” 
In another place (p. 45) in the same description he observes :—‘ The head (carapace) 
was formerly described by me as smooth, but in better specimens it is closely and fully 
sculptured, the plicee convex forwards.” It is evident that Mr. Salter here refers to this 
same specimen (PJ. XI, fig. 2); for although I have carefully examined all the heads of 
Pterygotus perornatus within my reach, I have been unable to detect any sculpture on the 
head-shield, nor, indeed, does it exist upon the carapace of this or of any other species of 
this genus. he fact, then, of the portion here referred to (fig. 2 4) being sculptured 
precludes us from considering it as the head-shield. With regard to the direction of the 
squamate markings having their “‘plicae convex forwards,” and the curves open backwards, 
as this would be obviously contrary to every other known specimen of Pferygotus 
showing sculpture, we are at once enabled to perceive that this portion belonged to the 
under side of the body of fig. 2. a, and has been squeezed forwards as well as much com- 
pressed. This explains the difficulty of the reversed direction of the squame. 
Mr. Salter has called the central appendage (figs. 2@ and 246 (e,c)) “the large 
ovate median lobe of the epistoma, shaped as usual.” This most satisfactorily settles the 
question of the nature of this anterior displaced organ of figs. 2a, 24. The plate at that 
time (1859) described as the epistoma, or conjoined epistoma and labrum, is now known 
as the thoracic plate or operculum; c is its central appendage, and op, op are the two 
broad lateral wings, closely covered with squame. 
But that which is of the greatest interest to us in this specimen (as bearing upon the 
question of the position of the branchiz), is that between the curved and squeezed-up 
* The portion so called is seen in our enlarged figure, Pl. XI, fig. 2d. 
2 See the lateral portions of fig. 2 6. 
3 Pl. XI, figs. 2a and 2 4, letter ec. 
4 Ibid., letters dr, dr, 
