BULLETIN OF THE UNITED STATES FISH COMMISSION. 199 



The dermal plates are regarded as ganoid by Owen. They appear to 

 me to be of sub-epithelial origin, as they are covered with an epithelial 

 layer of cells in the young, which persists in the adult, as in the case 

 of the outer covering of the scales of true ganoids {Lepidosteus)^ where 

 there is a very thin, soft external organic investment. They are some- 

 what irregularly conical in the young on the fore part of the body, as 

 shown in section on the head and back at sc. They are thickest at 

 their apices, and probably grow in thickness from below. On the top 

 and front of the head there are two pairs, on the back four pairs, on 

 the sides of the body one row of three on each side, and a transverse 

 row of three on each side in front of the dorsal ; behind the dorsal on 

 the tail, there are first two rows of four and then one row of two, so 

 that it is clear that a good many must be added to make up the number 

 of ])lates observed to cover the adult. The ventral row found on the 

 adult is absent in the young. Altogether there are more than three 

 times as many plates developed on the full-grown adult male of the same 

 species as are found in the young of the age here described. How these 

 are added can only be learned by further study of more material repre- 

 senting a greater number of stages. 



EXPLANATION OF PLATE. 



Young Hippocamims antiquorum viewed from the side as a transparent object, 

 enlarged 43 times. 



al), air bladder; af, anal fin; al, urinary vesicle or bladder; at, venous sinus; au, 

 auditory capsule ; h', h", h'", h"", first, second, third, and fourth branchial arches of 

 the right side ; ha, bulbus aortse; he, basiradial cartilages; 6/, breast or pectoral fin; 

 c, cerebellum; ce, cerebrum; ch, chorda dorsalis or notochord; cs, coraco-scapular 

 arch; ctj, ceratohyal cartilage; df, dorsal fin; e, inter-nasal cartilage ; Jihy, hypohyal 

 cartilage; km, hyomandibular cartilage; i, intestine; ic, interradial cartilages or basa- 

 ha of fin rays; iv, intestinal valve; la, labial or inter-maxillary cartilage; Iv, liver 

 m, medulla spinalis or spinal cord; m' mouth; mh, mid-brain; mk, Meckel's cartilage 

 mo, medulla oblongata; mt, metapterygoid cartilage; na, nasal i)it; o?, oesophagus 

 p, parachordal cartilage; pn, pineal gland; jnj, pituitary body; q, rod-like quadrate 

 cartilage; re, rostral cartilage or prolongation of the trabecular cornu; ri, rectal por- 

 tion of intestine; s, spiracular outlet of the gill-chamber; sc, dermal scutes or plates; 

 sy, elongated symplectic; t, trabeculse cranii seen from the side; ie, tegmen cranri; 

 V, vent or anus ; ve, ventricle of heart ; to, Wolffian or segmental duct ; x, supra-angular 

 cartilaginous element, the rudiment of the supra-maxillary. 



