io8 



NA TURE 



[y^nne ii, 1874 



one another below, and have grafted themselves above to 

 the auditory capsule, thus approximating very closely to 

 the state of things fuund in the ray, where, as in this 

 early stage of the salmon, the two parts of the hyoid are 

 nearly equal in size. The pterygo-palatine has not yet 

 united to the mandibular arch, although it has joined 

 anteriorly with a "conjugational process" sent out from 

 the now flattened trabecula. Meckel's cartilage is entirely 

 separated from the quadrate. 



The chief point to be noted in the fourth stage is 

 the assumption of an undoubted Teleostean character, 

 by the slipping down of the posterior bar of the 



FujW. 



Flf; 10— He.id of Embrvo Salmoi, about j inch long (x lo diam.). 

 H, heart. 



hyoid, which is now attached, not to the upper angle of 

 the anterior bar, but to about its middle, a small nodule 

 of cartilat;e, the inter-hyal, appearing between the two. 

 This important change has advanced still farther in the 

 fifih staice (Fit'. Ii), in which also the palato-pterygoid 

 h-i> iin'"ei "iih th-- quadrate, and the membranous roof 

 of 'he :ir .in-Ci^se, beginning to chondrify, his formed the 

 a- teiior part oi thf legmen cranu (T.Cr;, and sent back 

 a .-.up'a-o bual bar (S.Or) to meet the ear cnpsule, leaving, 

 however, a large membranous spare or fontanelle (Fo) in 

 the roof ot the cra'ium. The trabeciihi;, although flattened 

 out and united in front, are completely separated behind, 

 both from one another and from the investing mass, which 



Figtl. 



TXr^ 



r~~-^. Illy 



Fir, II.- Skull nf V.^ung SHlmr,-.. the serond week after hatching. (X 12 

 diam.) Fo, loi.lanelle : I.lly, inter-hyal. 



is merely overlapped by their slender inturned posterior 

 ends (pharyngo-trabeculars). The jaws are constituted 

 exclusively by the palato-pterygoid and Meckelian car- 

 tilages, and in many other points the skull now bears a 

 very close resemblance to that of the shark or ray, and 

 still moie to that of certain recent Ganoids, such as Poly- 

 pteius. 



The sixth stage shows ossification to have set in at 

 several points, and exhibits in an interesting manner the 

 formation of the inter-orbital septum. The cartilage 

 between the nasal sacs (mesethmoid) has sent backwards 

 a tr'angular plate towards the oibito sphcno'd.^l region, 



another plate has risen up from the middle line of the 

 skull-floor or coalesced trabecukt ; and by the subsequent 

 union of these two elements the partition so characteristic 

 of bony fishes, as well as of reptiles and birds, is pro- 

 duced. It is the fissure left by the incomplete union of 

 these elements which is shown at c.t.f in Fig 8 (p. lo). 

 In the seventh stage all the ossifications have appeared, 

 and the skull is fast taking on adult characters. 



V. Skull of the Axolotl (Siredon pisciforme). The 

 group of tailed Amphibia or Saurobratrachia is one of the 

 most interesting in a craniological point of view, pre- 

 senting, as it does, so great a variety of types, that while 

 the highest, such as the salamander, approach nearly to 

 the frogs and toads, the lowest, such as Proteus and 

 Menobranchus, have a chondro-cranium actually lower 

 than that of the lamprey. As a rule, indeed, the skulls 

 of those Saurobratrachia which, like the Axolotl and the 

 two genera mentioned above, retain their gills throughout 

 life, have, when once the investing bones are removed, 

 a simpler and more embryonic structure than that of any 

 other adult animal. 



The two chief roofing-bones of the brain-case — the 

 parietals and frontals— are far more normal in their 

 relative size than in the salmon, the parietals uniting 

 in the mid-line, and sending off an unusually long anterior 

 process to the ethmoidal region. The nasals are sepa- 



Tig.iZ. 



-Skull uf nearly adult Axolotl 

 stapcdio-suspensorial ligamei 

 , mandibjio-hyoid lisjam-nt 



H.Br.tJ. 



(.X2diim.) A.N, an:eri..ri arcs : 

 : h s I . hyo-suspensorial ligament : 

 St. stapes; G, girdle-bone; Sq. 



rated from one another by the long ascending processes 

 of the pre-maxilla : the supra-ethmoid of the salmon is 

 absent, but the lateral ethmoid is represented by a mem- 

 brane- bone (Fig. 1 2, L.Eth) evidently corresponding with the 

 pre-lron;alof reptiles, which overlies the cartilage behind 

 the nasal sac and extends backwards to meet the anterior 

 process of the parietal. The maxilla is considerably 

 smaller than the pre-maxilla, and is free behind, there 

 being no jugal or quadrato-jugal to unite it with the 

 quadrate. On the under-surface ol the skull is the large 

 oblong para-sphenoid, and in front of it, bounding the 

 inner side of the posterior nares. the wcU-developed 

 tooth-bearing vomers, which together represent the single 

 bone oflhat name in the salmon. All the opercular bones 

 of the fish arc absent, except the pre-opercular, now, as in 

 all the high vertebrata, known as the squamosal (Sq), a 

 flat ossification clamping the suspensory apparatus of the 

 jaw, and extending upwards and backwards to the auditory 

 region. 



In the mandible three membrane-bones are developed, 

 the two first of which bear teeth ; the dentary has the 

 same relations as in the salmon, the splenial lies as a 

 flat splint on the inner side of each ramus, and the angular 

 is also chiefly visible within, a small portion of it only 

 (Ang) being seen externally. 



The remaining bones will be described with the chondro- 

 cranium, of which they are ossifications. 

 {To be continue' J.) 



