122 



PALAEONTOLOGY 



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ventrals — these latter were first observed by Sir P. Egerton. 



The jaws are small, and possess confluent denticles. 



The type-species is the Pterichthys Mitten ; others have 



been based upon proportions of the cuirass, of the pectorals, 



and the tail ; all are from the " old red sandstone," and the 



great majority have been 

 found in the Devonian 

 strata of Ross-shire, Caith- 

 ness, and other Scotch 

 localities. 



Genus Cephalaspis 

 (Jcephale, head ; aspis, 

 buckler). — In this genus 

 the posterior angles of the 

 shield-shaped helmet are 

 produced backward in a 

 pointed form, giving to 

 the head the form of a 

 " saddler's knife ;" in other 

 respects the genus closely 

 resembles Pterichthys. 



Mr. D. Page has re- 

 cently acquired specimens 

 of Cephalaspis from Lan- 

 arkshire tale-stones, form- 

 ing the base of the Devo- 

 nian system, which show 

 a dorsal fin, pectoral fins, 

 and a large heterocercal 

 fin, besides a well-marked 

 capsule of the eye-ball. 



Fig. n. 



Pterichthys ; Plastron or Ventral Shield 



(Devonian), after Pander. 



Cephalaspis Mv/rchisoni occurs in the passage beds from the 

 Silurian to the Devonian systems. 



Genus Pteraspis. The buckler of Pteraspis truncatus has 

 been found m a Silurian stratum below the Ludlow bone bed ; 



