PLATE XXI. 



Fig. 



1. Specimen of Pterichthys productus, seen from the ventral surface; natural 



size. From Lethen, in the British Museum Collection (49189). 



2. Another specimen, natural size, showing three of the ventral plates of the 



carapace from the internal surface, the right pectoral appendage, the dorsal 

 fiu, and the tail pedicle. From Lethen, in the British Museum (50112). 



3. Natural internal cast, uncompressed, of the lateral plates of the left side of 



Pterichthys productus ; natural size. From a specimen from Cromarty, 

 in the Hugh Miller Collection, Royal Scottish Museum, Edinburgh. 



4. Body carapace with right pectoral appendage of a small specimen of Pterichthys 



productus, laterally compressed ; head and tail wanting ; natural size. From 

 Cromarty, in the Hugh Miller Collection, Royal Scottish Museum, Edin- 

 burgh. 



