Mr. W. K. Parker on the Skull of Salmo salar. 223 



much more distinct condition of the parts than in frogs at the same 

 stage ; the differentiation of the latter is obscure as compared with 

 the fish, and this not merely because of the quantity of pigmentum 

 nigrum in its tissues. 



Then, in addition to other causes of obscuration, the mouth of the 

 tadpole is strangely modified in harmony with its " suctorial" cha- 

 racter and affinities (showing a remarkable affinity to the mouth of a 

 lamprey), so that a whole system of cartilages has to be eliminated 

 from the lips before the mouth (proper) can be understood. The 

 labial system is slightly and slowly developed in the salmon, and its 

 mouth is thus much more in harmony with that of the embryo reptile 

 or bird than with that of the tadpole. 



After the simple stage is passed, the development of the facial 

 arches is very different in the two types — as different, indeed, as in 

 any two possible examples that could be given in the whole vertebrate 

 group. 



The facial arches behind the mouth now undergo segmentation — 

 first the hyoid, and then the mandibular. The hyoid is cloven from 

 top to bottom and also has a single distal piece separated off. 



At this stage we get an explanation of what is seen in certain rays, 

 where the hyoid suspensorium is permanently double ; and also ascer- 

 tain that this second postoral arch, which retains the anterior piece 

 in relation to the skull as the great "hyomandibular" pier, does not 

 need the saw of the transcendentalist to put it into proper relation to 

 its surroundings. Nature's invisible loedge has done what was needed, 

 and the supposed double rib turns out to be half a visceral arch. 

 On the whole, this second stage is extremely " Plagiostomous," for 

 the details of which I must refer to the main paper. 



While in the egg the head of the embryo is flattened, and so 

 twisted that one of the eyes (it may be the left or the right) looks 

 upwards towards the " chorion," the other having a visceral direc- 

 tion. 



The facial bars, at first having all a simple sigmoid form, rapidly 

 change towards the time of hatching ; and when the head gets free, 

 the cerebral vesicles speedily swell, taking on the form so familiar to 

 the embryologist ; and the head now gains the " mesocephalic 

 flexure." 



After this an approach is made to the Teleostean type of struc- 

 ture ; but this is not done at a stride. The intermediate condition 

 is thoroughly "Ganoid," and, happily, comes in to explain the related 

 structures of the older and newer "Orders." I am not aware that 

 any stage of the heart or of the intestines shows either the many 

 valves of the "aortic bulb" or the intestinal spiral valve ; this must 

 be seen to ; yet if these never show themselves in the "fry" of the 

 osseous fish, their absence does not affect the general skeletal mor- 

 phology. 



The salmon amongst fishes, like the fowl amongst birds, nevei 

 attains to the greatest degree of special class-modification; it re- 

 mains subtypical, with a dentigerous maxillary, a ductus pneumaticus, 

 a very chondrosteous state of the skull, and a very heterocercal tail. 



