424 MANUAL OF ZOOLOGY. 



typical flattened form of the rays, and the snout is elongated 

 so as to form a long, sword-like organ, the sides of which 

 are furnished with strong tooth-like spines. This constitutes a 

 powerful weapon, with which the Saw-fish attacks the largest 

 marine animals. 



Before leaving the Elasmobranchii, a few words may be said 

 as to their position in the class of fishes. From the cartilagi- 

 nous nature of the endoskeleton, and the similarity between 

 the form of their gills and those of the Lampreys and Myxi- 

 noids, the Elasmobranchii were long placed low down in the 

 scale of fishes, to which also the permanently heterocercal tail 

 conduced. When we come, however, to take into considera- 

 tion the sum of all their characters, there can be little hesita- 

 tion in placing the order nearly at the summit of the entire 

 class. The nervous system, and especially the cerebral mass, 

 is very much more highly developed proportionately than is 

 the case with any other division of the fishes. The organs of 

 sense are, comparatively speaking, of a very high grade of 

 organisation, the auditory organs being more than ordinarily 

 elaborate, the eyes being sometimes furnished with a third eye- 

 lid (membrana nictitans], and the nasal sacs having a very com- 

 plex structure. The structure of the heart agrees with that of 

 the Ganoids, and is a decided advance upon the heart of the 

 more typical bony fishes. Finally, the embryo, before its 

 exclusion from the egg, is furnished with external filamentous 

 branchiae, this being a decided approximation to the Amphibia. 



ORDER VI. DIPNOI ( = Protopteri, Owen). This order is 

 a very small one, and includes only the singular Mud-fishes 

 (Lepidosiren and Ceratodus) ; but it is nevertheless of great 

 importance as exhibiting a distinct transition between the fishes 

 and the Amphibia. So many, in fact, and so striking, are the 

 points of resemblance between the two, that until recently the 

 Lepidosiren (fig. 182) was always made to constitute the lowest 

 class of the Amphibia. The highest authorities, however, now 

 concur in placing it amongst the fishes, of which it constitutes 

 the highest order. The order Dipnoi is defined by the following 

 characters: The body is fish-like in shape. There is a skull 

 with distinct cranial bones and a lower jaw, but the notochord 

 is persistent, and there are no vertebral centra, nor an occipital 

 condyle. The exoskeleton consists of small, horny, overlap- 

 ping scales, having the " cycloid " character. The pectoral 

 and ventral limbs are both present, but have (in Lepidosiren) 

 the form of awl-shaped, filiform, many-jointed organs, of which 

 the former only have a membranous fringe inferiorly. The 

 ventral limbs are attached close to the anus, and the pectoral 



