1905.] OF ELASMOBRANCII FISHES. 47 



largeui' des arcs, la place des dents pharyngiales inferieures et 

 partiellement la place des dents pharyngiales snpeiieures sont 

 couvertes de tr^s petites ecailles inides et pointues, la pointe 

 dirigee en arriere." On Echinorhinus he remarks, "Appendices* 

 deux cotes 1", 2*^, 3®, 4*^ et cote exterieur 5^ arc, longs sans dents, 



long 6 mm." " Pas de dents pharyngiales inferieures a 



voir. Pas de dents pharyngiales superieures a voir." 



In his account of the primitive Shark ChlamiydoselacJius an- 

 guineus, Garmau f mentions that both the mouth and throat 

 of that fish are coveied with scales which are largest on the inner 

 edges of the gills. 



It will be noted from the foregoing account that the presence 

 of denticles in the lining of the mouth and pharynx is of very 

 wide distribution among the Elasmobranchii. Out of the nine- 

 teen species which I have examined, only five were found to be 

 totally devoid of them. If there be added to these the species 

 examined by Steinhard and Popta we have a total, with Chlaniydo- 

 selachus, of thirty-two species, out of which only nine (or about 

 28 7o) have no denticles whatever. 



The facts and conclusions that are to be gleaned from a study 

 of these denticles may be summarised as follows : — 



1. In Heptanchus cmereus, Chlamydoselachus anguineus, Mus- 

 telus Icevis, Gcdeus canis, Sphyrna malleus, Lamna cornuMca, and 

 (probably) Rhinohatusproductus, denticles are uniformly distributed 

 over the whole of the mucous membrane of the mouth, pharynx, 

 and branchial arches, and extend backwards to the commencement 

 of the oesophagus. Since this condition is met with in such gene- 

 ralised types as^the two first named genera, there is good reason 

 to believe that it represents the primitive method of distribution 

 which has been inherited by them from the ancestral forms of 

 existing Elasmobranchs. It seems probable that the variations 

 in the distribution of the denticles which are met with in other 

 species have been derived from this condition through their 

 becoming restricted to certain areas only. The first and simplest 

 modification is exhibited in AcantMas vidga?Hs. In tbis species 

 the denticles are wanting from the roof of the mouth and pharynx. 

 In Alopecias vulpes these structtu-es are absent from both the 

 roof and floor of the mouth and pharynx, and hence they are 

 restricted to the pharyngeal mai'gins of the branchial arches. In 

 Rhina squatina they have ceased to be developed in the pharynx 

 except on the mucous membrane covering the hj^oid and first 

 branchial ai-ches. They are retained, however, over a considerable 

 area on both the roof and floor of the oral cavity. In Scyllium 

 eanicula, Echinorhinus spinosus, Myliohatis tiquila, Tmpedo 6'cel- 

 tata, and Trygon walga denticles have become lost altogether. 



2. In all cases where I have examined the denticles micro- 

 scopically, their structure and form proved that they were un- 



* I. e. gill-rakers. 



t Bull. Mus. Comp. Zool. Harvard Coll. vol. xii. No. 1, 1885, p. 2, pi. v. ' 



