340 Prof. Morns — On the Genus jEchmodus. 



fishes, -with, rhomboidal scales, a single well-developed dorsal fin, 

 partly opposed to the anal, the pectoral and ventrals being small, 

 and the fin-fulcra in a single series. They closely resemble Tetra- 

 gonolepis and Dapedius in general form and character, but differ from 

 the latter genus, as above noticed, in having the anterior teeth conical 

 and single pointed, instead of being notched or bifurcate ; from the 

 former they are distinguished by the dentition and mode of articula- 

 tion of the scales. 



On the discrimination, however, of a genus of fishes by the form of 

 the teeth. Sir P. Egerton remarks : — 



" But, alas for the constancy of fishes' teeth ! a specimen came 

 into my hands not long ago having a combination of the two forms 

 of tooth, the principal sets in each jaw being conical and single- 

 pointed, and all the subsidiary teeth bifurcate. Having had my 

 attention thus directed to this point, I have since found a specimen 

 of Dapedius punctatus in Lord Enniskillen's collection, which has 

 both forms of tooth in the principal series in both jaws. The con- 

 clusion, therefore, is irresistible, that the form of tooth is a character 

 too capricious to be relied upon in this instance as a generic 

 definition." ^ 



The species of ^chmodus are chiefly from the Liassic deposits of 

 Europe, and one has been recognized from the Oolitic beds of the 

 Deccan, the Dapedius Eger'toni, Sykes [^climodus, Egerton).^ 



It has been considered useful to figure this species of JEchmodus, 

 not only as illustrating the genus which has not heretofore been 

 figured in any English work, with the exception of a restored 

 outline of the genus in " Lyell's Elements of Geology," p. 418, but 

 also as shewing a type of ganoid fish of the Mesozoic period 

 having the tail nearly symmetrical (homocercal) — a character by 

 which the majority of the Secondary ganoids are distinguished from 

 the so-termed heterocercal ^ ganoid fishes of the Palaeozoic period. 



1 Quart. Journ. GeoL Soc, Vol. ix. p. 275. 2 j^ia. p. 352. 



3 With regard to these terms, see the remarks b)' Prof. Haxley, Quart. Journ. Mic. 

 Science, Oct. 1858, and Mem. Geol. Survey, Decade x. to p. 3, where he states " that 

 the so-called ' homocercal' Teleostei of the present epoch are in reality excessively 

 heterocercal ; but the word 'homocercal' is now so generally understood to signify a 

 tail like that of most existing Teleostei, tlmt I prefer to employ Prof. M' Coy's term 

 ' diphycercal' for truly homocercal tails." 



In alluding to these structures, Prof. Owen writes : — •" The shape of the caudal 

 fin varies much in fishes, according to the kind and degree of motion required : 

 in the imprisoned embryo, in the long and slender undulating eel, in the sluggish 

 Lepidosiren, the vertebrae continue to the end of the body in a straight line, 

 distinct and decreasing to a point; and the tail is bordered above and below by 

 a vertical fold of skin ; terminating either in a point or obtusely. Such fold or 

 fin is symmetrical, but not ' homocercal.' The vertical folds deepen ; at first 

 equally, forming a terminal lobe ; then excessively, in the lower or haemal fold, 

 with the developement therein of rays, and with an upward or neural inclination 

 of the supporting vertebrae. Shorter rays are developed in the shallower neural 

 fold, which terminates at the pointed end of the vertebral series. The anterior 

 rays of the haemal fold, which are the longest, form a second point. The tail 

 is thus bifurcate, but unsymmetrical ; and this stage of the developement is termed 

 the ' heterocercal ' one. It was the fashion of tail which prevailed in fishes through- 

 out the palaeozoic and triassic periods. In some oolitic fishes, first is observed such 

 a lengthening of the dermoneurals of the tail, with such a shortening and run- 



