CEPHALASPID^. 



41 



scales becoming overlapped to a greater or less extent, accord- 

 ing to the angle assumed. Probably each scale indicates a 

 sclerotom. 



The fins are two pectorals of very peculiar form and character, 

 a dorsal placed posteriorly and a caudal. The dorsal and the 

 caudal were known to Prof. Agassiz ; but the pectoral appendage, 

 though indicated in Mr. Dinkel's drawing in the 'Poissons 

 fossiles' of the Glaramis specimen (which has since been chiselled 

 so as to obhterate these organs), are not described in the letter- 

 press. Mr. Powrie called attention to these strange jjcctoral 

 organs in the ' Geologist,' 1861, p. 137, and they are well seen 

 in some of his beautiful specimens illustrated in the plates of 

 this work. They differ from the other fins in presenting no 

 trace of fin-rays or of the fine scales which extend upwards 

 from the base of the dorsal and caudal. They are simply ellip- 

 soid expanses, with some calcareous matter in their structure, 

 which has caused them to be preserved, and has rendered 

 visible, especially in fig. 1, PI. X, a kind of reticulate or areolate 

 markings, quite peculiar to them. The character of these 

 fins differing so much from that of the other fins tends to 

 suggest that they may have had other functions than that of 

 mere locomotion, and it seems not at all improbable from 

 their position that they may have been efficient in causing 

 currents of water to pass to the branchial organs covered in 

 by the great head-shield (whose outlets are indicated by the 

 lateral perforations in the shield of Pterasjns), and have thus 

 aided respiration as well as locomotion, as is observed in the fry 

 of Teleostean fishes at the present day with regard to the 

 pectoral fin. These pectoral organs are mentioned here espe- 

 cially because it seems hardly doubtful that they are charac- 

 teristic of the group, possibly also of the Helerostraci ; it 

 is not likely that they were developed in but one single genus 

 of the closely allied series classed as Osteosfraci. 



A very remarkable specimen of shagreen-like structure has 

 been discovered by ]\Ir. Powrie in Forfarshire, in beds which 

 have furnished Cep/talaspis. As I have not been able to 

 assign it to Cephalaspidian fishes, though it may possibly 

 be connected with them, I only allude to it here. It consists of 

 a surface covered with minute spinous tubercles, the whole having 

 the appearance of a fossilized piece of shagreen, and its shape 



Fig. If 



ephalaspis 



