58 PROF. T. H. HUXLEY ON CERATODTJS FORSTERI, [Jan. 4, 



recollected that known forms certainly represent but a portion, and 

 probably a small portion, of those which have existed, and that the 

 most natural groups are therefore, to a great extent, the result of the 

 influence of extraneous, and what may be properly termed accidental, 

 conditions. 



It has occurred to me that, in the present state of science, it is 

 very desirable to have some mode of stating the facts of morphology 

 in a condensed and comprehensible form, which shall be purely ob- 

 jective and free from speculation ; and I now proceed to illustrate my 

 meaning by drawing up a scheme of the morphology of the 

 Ichthyopsida. 



Looking at the animals included under this head as a whole, or at 

 the development of any of the higher members of the group, it is 

 observable that they present a certain series of stages of differentia- 

 tion marked by the broad characters of the skull, the nature of the 

 olfactory and respiratory organs, and the development or non-develop- 

 ment of an opercular fold of the integument. 



Thus the skull either retains its primitive segmentation (Entomo- 

 crania), or the primitive segmentation is lost, and a chondrocranium 

 is developed (Holocrania). There are two external nostrils (Am- 

 phirhina) or only one (Monorhina). 



A. 'pneumatocele, or air sac, which may become either an air- 

 bladder or a lung, is developed (Pneumatoccela), or not (Apneumaio- 

 ccela) ; and a fold of the integument may cover the branchial aper- 

 tures (Operculata), or not (Inoperculata). 



The Ichthyopsida also exhibit a series of stages of differentiation of 

 the limbs, being either apodal or pedate ; and, when pedate, having 

 the limb-skeleton constructed upon the type of the archipterygium, 

 or on that of the icthyopterygium, or on that of the chiropterygium. 



Moreover, when the limb is an ichthyopterygium, it may possess 

 one, or at most two basal elements, which articulate with the pectoral 

 arch (unibasal), or there may be three (tribasal), or there may be 

 many (multibasal), in accordance with the greater and greater diver- 

 gence of the fin from the archipterygial type. 



The chondrocranium may be constructed upon either the amphi- 

 stylic, the hvostylie, or the autostylic plan. 



Now, if the stages of general differentiation be indicated by points 

 on a vertical line from which horizontal lines are drawn, and the 

 stages of subordinate differentiation of the skull and limbs be indi- 

 cated by points on a horizontal line from which vertical lines are 

 drawn, we shall have vertical series of intersections indicating general 

 differentiation, and horizontal series of intersections indicating special 

 differentiation. Every known form will occupy some given inter- 

 sections, and the unoccupied intersections will indicate unfulfilled, or 

 unknown, possibilities of organization. 



The following Table exhibits the groups of the Ichthyopsida 

 arranged according to this scheme. 



