144 LECTURE VI. 



testaceous and crustaceous Invertebrates appears to have been the 

 rule in the first-born fishes of our globe. 



These primoBval members of the vertebrate sub -kingdom manifest 

 other remarkable traits of embryonic life. The Cephalaspids of the 

 Old Red Sandstone were shaped like the tadpoles of Batrachia ; the 

 breathing organs and chief part of the alimentary apparatus were 

 aggregated with the proper viscera of the cranial cavity, in an 

 enormous cephalic enlargement ; the rest of the trunk was for loco- 

 motion, and dwindled away to a point. The cephalic abdominal 

 enlargement was defended by large bony scutes ; the muscular tail- 

 part was, in the higher species (Coccosteus), strengthened by an in- 

 completely developed vertebral axis, with intercalary and dermal 

 spines, supporting a dorsal and an anal fin. The position of the anal 

 fin proves the anus to have been situated, as in tadpoles, im- 

 mediately behind the cephalic abdominal expansion. In the lowest 

 forms, as Pterichtlri/s, the mouth was small and inferior, as in the 

 young tadpole, and the post-cephalic or abdominal part of the en- 

 largement very short and ill-defined. In the Coccostens it nearly 

 equals the cranial part of the enlargement ; the scutes are fewer, 

 larger, and show the progress of coalescence ; the mouth is anterior, 

 large, and formed by well developed dentigerous upper and lower 

 jaws. In this genus the cephalic or opercular appendages are in- 

 conspicuous or reduced to the normal proportions ; in Pamplir actus 

 and Pterichthys they form long fin-like appendages, projecting from 

 the sides of the cephalic enlargement, like the external gills of the 

 Batrachian and Selachian larvas, and they may have supported 

 external fringed gills in the ancient Cephalaspids. 



Genesis of FIjis. — In the order of succession of Fishes the develop- 

 ment of locomotive organs is first restricted, as in most Cephalaspids, to 

 the region of the head : in Pterichthys and Pamfhracttis they project 

 like pectoral fins (whicli M. Agassiz describes them to be) from the 

 sides of the head just anterior to the division between the facial and 

 nuchal plates, and from the place corresponding to that occupied by the 

 pedicle of the lower jaw, from which the opercular fin projects in the 

 Sturgeon. There is no trace of true pectoral, ventral, or of vertical fins 

 in these Cephalaspids. In the Coccosteus these cejjhalic fins are reduced 

 to ordinary opercular proportions (they appear to be represented by 

 the plate b in the restored side view, given by M. Agassiz, Op. cit. tab. 

 xxiv.) ; but here we have the earliest manifestation of dorsal and anal 

 fins, without, however, any modification of the terminal vertebra; to 

 form a caudal fin, either heterocercal or homocercal, and without the 

 slightest trace of true pectoral or ventral fins. In the Dipterus and 

 Glyptolepis tlierc are two closely approximated dorsal and two anal 



