The Embryology of Chlamydoselachus 601 



nearly meet across the isthmus, presaging the condition that suggested the name of our 

 fish — the cloak'gilled shark. 



Lateral Aspect, Full Length. — Comparison from this viewpoint of the 34'mm. 

 embryo (Figure 23, plate II) with that of the 20'mm. specimen (Figure 18, plate II) shows 

 clearly that both differentiation and growth have taken place. The head has rounded out 

 and is almost protuberant, the mouth is almost closed and looks like a mouth. The gill' 

 slits have lost the embryonic look even though short filaments are present. The paired 

 fins show progress. The pectoral'fin base has some of the radial cartilages faintly shown. 

 The pelvic fin is well differentiated. Even more differentiation is seen in the tail parts. 

 Noticeable is the development of the dorsal and anal fins and of the dorsal and ventral 

 lobes of the caudal (the latter lobe being better developed). This caudal, however, has 

 an even more marked downward swing than that of the 20'mm. embryo. The original 

 drawing of this figure measured 223 mm. 



Comparison of this 34'mm. frilled shark in lateral aspect (Figure 23, plate II) with 

 Scammon's 34'mm. dogfish in like aspect (his Fig. 31, pi. IV) shows that the dogfish is 

 further advanced in development than the frilled shark. The fins (both median and paired) 

 of Squalus are much better developed, and from its spiracles and gill'arches extend a great 

 profusion of long external gills. Its lateral'line system is more prominent. The mouth of 

 Chlamydoselachus, however, is far better developed than that of Squalus. 



An Embryo of 39 Millimeters 



Of this specimen. Dean's "List" calls for drawings, ''Entire in three positions". 

 This is the first embryo of which there are three full'length portraits. There are four 

 other embryos each drawn full'length in three aspects — dorsal, lateral, and ventral. For 

 all five embryos, these full'length drawings will be studied in the order just noted, and the 

 embryo in each aspect will be compared with the next younger embryo in the like aspect. 

 Studied in such order one will get the most comprehensive view possible of each stage. 



Dorsal Aspect. — The original drawing of this embryo measures 258 mm. and the 

 magnification is 6.6. As seen in Figure 26, plate III, this is a trim'built embryo. The 

 head is much rounder in front than that of the 34'mm. embryo shown in Figure 22, plate 

 II, but it is still wide between the eyes. Forward of the first gill'flaps, are found the large 

 spiracular clefts. The gill'folds are all well developed — the first markedly so. The 

 pectoral fins are still in about the same stage of development as was found in the 34-mm. 

 specimen. The pelvics, however, show up plainly alongside the slender body. The 

 dorsal fin and the upper lobe of the caudal are faintly outlined . 



Lateral Aspect. — In making this drawing of the 39"mm. embryo (Figure 25, plate 

 II), the artist availed himself of artistic license to the amount of 10 mm. over the preceding 

 figure — the original drawing measuring 268 mm. This specimen will now be compared 

 with the 34'mm. fish in similar aspect (Figure 23, plate II). In the drawing it is seen that 

 the head in front of the eyes has elongated somewhat. The olfactory organ has moved 

 forward with reference to the eye- -which no longer shows the choroid fissure. The 



