294 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 25, 



in breadth and 2| lines in length ; telson fragmentary, no donbt 

 ensiform when properly preserved. 



4. Ptertgottts eaniceps, sp. nov. PL IX. fig. 3. 



Head-shield obtusely triangular. Eyes marginal. Breadth of 

 posterior border of carapace 6 lines ; greatest length 4| lines, from 

 posterior border to eyes 3 lines ; breadth of carapace at eyes 3^ lines ; 

 diameter of eye 1 line. A small depression marks the centre of the 

 carapace between the eyes ; and a small elevation may be seen 

 slightly posterior to them, also on the median line. 



Length of six segments 7| lines, greatest breadth 9 lines ; no 

 ornamentation preserved. 



No ocelli are visible ; the eyes, however, are very prominent. 



Farther Observations on the Structure of Pterygotus. 



In Messrs. Huxley and Salter's Monograph, already alluded to, 

 there is figured on pi. 1. fig. 16, a portion of a Pterygotus (referred 

 by Mr. Salter to P. perornatus, var. jplicatissimns) exhibiting the 

 squeezed and distorted carapace and anterior segments with the 

 thoracic plate and its characteristic median lobe, " showing," says 

 Mr. Salter, " what are perhaps the traces of the impressions of 

 muscles for the ectognaths." The specimen is preserved in the 

 Museum at Jermyn Street, where, by Professor Huxley's kindness, 

 I have been permitted to examine it. 



I have no doubt that we have here evidence of true branchise ; 

 and several other specimens which have since been obtained from 

 Lanarkshire tend to confirm this view. 



The specimens show a series of leaf-like bodies presenting a 

 highly vascular and, at the same time, delicate structure, arranged in 

 linear series of from six to eight in each row, and appearing to have 

 occupied a position beneath the thoracic plate on the ventral surface 

 of the body, as seen in Limulus at the present day (see PL X. 

 figs. 3 a & 3 6). The form, however, of the branchial plates in Ptery- 

 gotus is quite dissimilar to those in Limulus. 



In the latter the vascular striae are parallel to the circumference 

 of the lamella of the giUs ; in the former they branch and subdivide 

 from the centre to the margin of the plates, becoming finer and 

 more numerous on the border. 



Length of plates If inch long, by 9 Hues in breadth. 



Pterygotus hilobus, Salter*. This was formerly, and still con- 

 tinues to be, the most abundant species from Lanarkshire, and is 

 frequently met with in a very perfect state of preservation. 



It obtained its specific name on account of the bilobed character 

 of the telson ; but at that time no entire example of Pterygotus peror- 

 natus had been met with. 



P. perornatus has now been obtained in a perfect state and is 

 also furnished with a bilobed telson. 



* No figures are giren here of the varieties of Pterygotxis bilohus, as they will 

 be delineated on the plates of the forthcoming number of the Palaeontogrnpliical 

 Society's Monograph on the Merostomata. 



