248 NATURAL SCIENCE. April. 



Figs. I, 2, 3. In Fig. 2 the body-width has purposely been made 

 ■narrower than in the fossils, and in Fig, 3 an attempt to present 

 an idea of the head-view is believed by the writer to be trust- 

 worthy as to the down-turned position of the pectoral fins, and 

 in general as to the vertical proportions. Outwardly, the subfusi- 

 form body would appear to resemble closely that of a modern 

 shark ; the fins, too, in their size and position, have somewhat 

 of a modern look, and at the base of the tail occurs the small 

 horizontal dermal keel of many living forms. Its paired fins served 

 unquestionably as balancing organs, and could have had but little 

 movement save at right angles to the plane of the fish's motion. 

 The tail, homocercal in outline, and with its sharply-marked lateral 

 keels, is too nearly of the outward character of the mackerel's to 

 permit any other belief than that the swimming of the fish was active 

 and rapid, and its shape certainly destroys the hypothesis of Jaekel, 

 that Cladoselache was a form specialised to bottom-environment. 



That Cladoselache was an exceedingly generalised shark, and that 

 it, in fact, gives at the present time our clearest idea of the characters 

 of the ancestral Elasmobranch, seems to be warranted by the study 

 of the following structures : — 



[a.) Notochord, Vevtehvd Arches. — From the mid-body region 

 backward, a broad notochordal space may be traced in favourable 

 fossils {e.g., one discovered by Rev. Dr. Kepler, and given by him 

 to the college at Delaware, O.) ; in the tail this is found to be 

 underlaid by a well-defined tract interpretable as sub-notochordal 

 rod. The neural and haemal arches are also of interest ; they prove 

 to be of moderate size and thickness, extending no further outward 

 than about one-half the vertical distance between notochord and 

 integument. They are metameric in arrangement, i.e., corresponding 

 in number with the calcified remains of the muscle-plates, and closely 

 resemble each other — each a tapering rod of cartilage forked at its 

 base. Interneurals are absent, thus distinguishing Cladoselache from 

 the Pleuracanthidae and all other sharks. 



{h.) Branchial Arches, Jaws. — Five gill-arches are clearly present 

 '{cf. Professor Claypole, American Geologist, vol. xv., Jan., 1895), ^^^ ^" 

 a favourable specimen in Columbia College there appear to be traces 

 of a sixth and seventh ; they are interesting in their similar size, their 

 stoutness, and in the transverse plane in which they lie. Their 

 elements have not been clearly distinguished. The upper and lower 

 jaws are similar in shape and size ; their hinge is supported by 

 an elongate hyo-mandibular, somewhat as in Chlamydoselache or 

 Scyllinm. In the wide space between this element and the palato- 

 quadrate a spiracle may well have been present. The mouth was 

 terminal ; the gills possessed frilling integument, supported by fibrous 

 rays, and the foremost of these dermal gill-frills appears to have been 

 sufficiently large to have served as an operculum. 



{c.) Integument, Teeth. — The integument, everywhere lacking in 



