﻿PTERYGOTUS BILOBUS. 67 



But there is yet another specimen, which seems to complete the evidence of the 

 position of these branchial plates in Pterygotus. 



I allude to a specimen described in the ' Memoirs of the Geol. Survey,' Monograph I, 

 p. 49, under the name of Pt. perornatus, var. plicatissimus. One of the body-rings is 

 there figured, loc. cit. ; and the anterior portion of the specimen is represented in pi. i, 

 tig. 1 6, of that work. We have figured the whole of the same specimen of the natural size 

 in PI. XI, fig. 2 a, and the anterior portion enlarged twice the natural size in fig. 2 b. 



Mr. Salter writes as follows : — " The carapace 1 is much compressed longitudinally, but 

 the true form would probably be a full semioval ; the position of the eyes is obscurely 

 marked. The surface is covered with semicircular plicae, 2 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 4 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 plicae convex forwards." It is evident that Mr. Salter here refers to this 

 same specimen (PL 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. The fact, then, of the portion here referred to (fig. 2 b) 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 Fterygotus 

 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 squamae. 



Mr. Salter has called the central appendage (figs. 2 a and 2 b (c, 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. 2 a, 2 b. 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 squamae. 



But that which is of the greatest interest to us in this specimen (as bearing upon the 

 question of the position of the branchiae), is that between the curved and squeezed-up 



1 The portion so called is seen in our enlarged figure, PI. XI, fig. 2 b. 



2 See the lateral portions of fig. 2 b. 



3 PI. XI, figs. 2 a and 2 b, letter c. 

 * Ibid., letters br, br. 



