270 THE OLD MOUTH AND THE NEW. 



the haemal surface at the base of the caudal fin. (Fig. 248.) This fold is usually 

 sharply defined and shows no trace of subdivisions or of supporting rays. 



In a small undescribed species of Cephalaspis from Dalhousie, N.B., a similar 

 fold is present, but it appears to be strengthened by minute, ill-defined spicules. 

 (Fig. 234.) 



In the larger species, like C. lyellii and C. murchsonii, in place of a membran- 

 ous fold, there is a series of separately movable processes, segmentally arranged 

 and covered with a dermal skeleton having the same surface ornamentation as 

 that on the rest of the body. There is nothing in true fishes exactly comparable 

 with these remarkable structures. 



In C. lyellii (Fig. 232), the fringing processes hang freely away from the trunk, 

 in a nearly vertical position, with their distal ends bending backward in graceful 

 curves. Each process has a slender neck and rounded head that fits into a cup-like 

 depression on the posterior ventral margin of the large dorso-lateral trunk scales. 

 (Fig. 233, C.) There are from twenty to thirty pairs, beginning just back of the 

 cephalic shield and gradually decreasing in size from that point toward the tail 

 end. The most posterior ones are reduced to mere spines or rhomboidal plates, 

 loosely articulated to the lateral trunk scales. 



In C. murchisonii the fringe plates are distinctly lobed, and overlap one an- 

 other so that their flattened surfaces are directed diagonally forward and outward. 

 (Fig. 233, D.) In Cephalaspis pagei they appear to have a similar shape and 

 arrangement, but are armed with coarse projecting spines that give them a 

 decidedly arthropod appearance. (Fig. 233, E.) 



There can be no doubt that the fringing processes represent an initial stage 

 in the formation of a lateral fold like that in the embryos of true fishes. They 

 may be regarded as small segmentally arranged locomotor appendages compar- 

 able with arthropod abdominal appendages, but belonging to metameres lying 

 farther back than any that occur in that class of animals. An alternative inter- 

 pretation would be to regard them as a series of overhanging pleurites like those in 

 the trilobites aftd in many other arthropods. 



We may therefore conclude that the oral arches, gill arches, external gills, 

 "balancers," the lateral folds of vertebrate embryos, the cephalic oars and fring- 

 ing processes of the ostracoderms, are various local modifications of one set of 

 serially homologous structures, that are in turn comparable with the segmental 

 appendages of arthropods. The pectoral and pelvic fins of true vertebrates are 

 to be regarded as comparatively recent modifications of the lateral folds, and 

 as containing the remnants of a large and varying number of such appendages. 



This interpretation of the origin of the paired appendages gives us precisely 

 what Gegenbaur claims has heretofore been lacking in the lateral fold theory, 

 namely: i. a reason for the existence of the primary fold of ectoderm that initiates 

 the formation of the lateral fold; 2. a reason for the migration into it of segmental 

 detachments of muscle, nerve, and cartilage; and 3. a primary function for the 

 lateral fold, out of which a set of locomotor organs could be logically developed. 



