436 



Mr. W. K. Parker. On the 



[May 27, 



need he, worked out, either in the skull or in any other part of their 

 organisation. 



That which calls for it here is the great and unlooked-for poly- 

 morphism of the species ; I may explain this by saying that the skull, 

 in really important modifications, differs more in the species of some 

 of the genera than it does in the orders of some of the classes. As an 

 instance, it would be no easy thing to find a malacopterous fish differing 

 from an acanthopterous type, in deep-seated essential matters, so much 

 as the common toad does from the other native species, viz., the Natter- 

 jack ; and the common frog has only about half as many cranial 

 elements as the bull-frog of North America. 



If the metamorphosis of a single species be worked out exhaustively, 

 it gives a range of structural characters which rises up from a larval 

 creature on the level of the lamprey, to a reptilian form, not far 

 below the Chelonia, and evidently related (obliquely, not genetically) 

 to that "order." 



Moreover, whilst the " opisthoglossa " have larvae with suctorial 

 mouths, and a guasi-petromyzine structure altogether, the larvae of the 

 " aglossa " need only to be arrested as larvae, and to acquire a dense 

 bony armature to be very close counterparts of the most bizarre forms 

 of the ganoids of the "old red sandstone," such as Pterichthys and 

 Coccosteus. 



The Batrachia show some remarkable things in their metamorphosis, 

 both as to the size their larvae obtain, and the time during which 

 metamorphosis is taking place. 



In the bull-frog (Bana jnpiens) the larvae attain the length of about 

 5 inches, and take two or three years for their transformation ; they 

 may be hindered in this, and be made to take twice that time. In 

 these the larvae bear a moderate relation, as to size, to the adult 

 form, which may be 7 inches long, although tail-less. 



But in a frog from the neotropical region (Pseudis) scarcely larger 

 than our native- form, the tadpole attains the length of nearly a foot, 

 the tail acquiring a breadth of 4 inches. 



As zoologists well know, it is easy to procure tadpoles of this species, 

 but very hard to get an adult. I am of opinion that the adult con- 

 dition is not attained until after many years ; and it suggests itself to 

 me that this species may be the not remote descendant of a type which 

 did not finish its anural metamorphosis. 



On the other hand, some of the neotropical forms have very small 

 tadpoles. Bufo chilensis, a large toad, has them about half the size of 

 those of our common native Batrachia, and the newly-metamorphosed 

 individuals are no larger than a house-fly. 



But in Pipa the small larvae are thoroughly metamorphosed in the 

 maternal dorsal pouches, and at first only do they show a trace (and 

 only a trace) of branchial tufts. 



