30 Proceedings of the Royal Physical Society. 



anterior ventro-lateral and the position upon it of the pectoral 

 articulaiion. 



The aiticular fossa on the outer side of the anterior ventro- 

 lateral in Pterichthys, with its contained helmet-process 

 grasped by the articular plates of the arm, and the foramen 

 for the passage of the vessels and nerves to the same, seems 

 to be conformed exactly as in Aster olepis ; and as these parts 

 have been so well described by Pander from Eussian speci- 

 mens of the latter genus, it is needless at present to enter 

 into detail respecting them. If the Scottish and Eussian 

 genera are distinct, the diagnosis must be founded on some- 

 thing else than the articulation of the limbs. 



Thirdly, as to the articulation of the body-plates with each 

 other. Sir Philip Egerton states that " all the plates of the 

 carapace, with the exception of the lozenge-shaped plate g 

 (of the under surface), are united by simple sutures ; this, on 

 the contrary, is attached to its neighbours by broad squamous 

 sutures, the lateral bones overlapping its margins on all 

 sides " (8, p. 306) ; but in the same paper he quotes Hugh 

 Miller to the effect that the two median dorsal plates over- 

 lapped some neighbouring ones, and were themselves over- 

 lapped by others. Now my observations show that all the 

 plates of the carapace were connected with each other by 

 overlapping or squamous sutures, a marginal band along the 

 internal surface of the overlapping plate being thinned off 

 to fit on to a corresponding band along the margin of 

 the outer surface of the one overlapped. The hexagonal 

 anterior dorsal plate {a. d.) in this way overlaps the ante- 

 rior dorso-laterals, but is itself overlapped along its 

 postero-lateral margins by the posterior dorso-laterals, and 

 also behind by the posterior median dorsal, though in 

 this latter case the contrary is stated by Hugh Miller (ib., 

 p. 309). 



The anterior' dorso-lateral (a. d. I.) overlaps the posterior 

 dorso-lateral, but is itself overlapped by the anterior median 

 dorsal and by the anterior ventro-lateral. 



The pfosterior dorso-lateral (p. d. I.) overlaps the anterior 

 median dorsal, but is itself overlapped by all the other plates 

 with which it is in contact, viz., the posterior median dorsal, 



