104 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Jan. 8, 



ichthys discovered by Mr. Baxter in the Farlow sandstone was penned, 

 the researches of Mr. Eoberts have brought to light from the same 

 locality several additional specimens of the same species, which 

 enable me to add the description of the ventral and thoracic plates. 

 The former specimen is still so far unique that it is the only one yet 

 discovered which gives a view of the dorsal surface, or reveals the 

 proportions of the head, from which the specific title was derived. 

 One of the more recently found specimens is quite a gem. The fish 

 reclines upon its back, and thus presents to view the ventral plates, 

 the thoracic plates, and their appendages ; the head and tail are 

 both wanting. See woodcut, fig. 2, and PI. III. fig. 8 & 9. 



Figs. 2 & 3. — Outlines of Specimens of Pterichthys macroeephalus 

 from Farlow. (See PI. III. figs. 8 & 9.) 



Fig. 3. 



In a former paper, read before the Geological Society in April 1848 

 (Quart. Journ. Geol. Soc. vol. iv. p. 302), the arrangement of the 

 plates composing the integument of this genus was so fully described 

 that it is needless to go over that ground again. I wish, however, 

 to correct an error in the number of the ventral plates. Two plates 

 are there enumerated as the posterior ventral plates, lettered h h 

 on the outline-figures, ibid. p. 305, which (as shown by Pro- 

 fessor M'Coy) are not independent elements of the shield, but 

 prolongations of the posterior ventre -lateral plates. I was led 

 into this mistake by the semblance of a suture visible on most 

 specimens, which proved to be the impression of the posterior mar- 

 ginal rim which encircles the inner posterior edge of the dorsal 

 plates, but traverses the inner surface of the posterior ventro- 

 lateral plates in the direction of the supposed suture. The im- 

 pression of this marginal rim is distinctly preserved in the Farlow 

 specimens (figs. 1, 2, — I), and affords a secure datum for measuring 

 the dimensions of the plates. The antero- posterior dimensions of the 

 dorsal surface were taken from the front of the first dorsal plate to 

 the posterior marginal rim ; a similar measurement of the ventral 

 surface, namely from the anterior margin of the shield to the impres- 

 sion of the posterior marginal rim, exactly coincides with the former ; 

 the width of the body and the length of the arms also correspond so 

 exactly that the two specimens might have been derived from the same 

 individual. The hinder prolongations of the posterior ventro-lateral 

 plates extend in this, as in all other species, beyond the termination 

 of the dorsal shield. In front of the anterior ventro-lateral plates 



