NATURAL SCIENCES OF PHILADELPHIA. 287 



Ascoceras. Ptychoceras. 



/?/?. Folded portions not in contact. 

 ? ? Hamites. 



atta.. One extremity spirally wound, the volutions not in contact. 



/?. Extremity of the shell prolonged beyond the wound portion. 



Lituites. Ancyloceras. 



/?/?. Extremity not prolonged in a line. 



y. The spiral flat. 



Gyroceras. Crioceras. 



yy. The spiral elevated (heliciform). 

 Trochocerus. Turrilites. 



otaa*. Spiral turns of the shell in contact. 



/?. Extremity prolonged in line beyond the spiral. 

 * * Scaphites. 



/?/?. Extremity not prolonged beyond spiral. 

 Nautilus. Ammonites. 



We may now consider the question of the origin of these higher groups. 

 In the first place, we must lay down the proposition that (he characters which 

 constitute groups ^'■higher" in the comparison of rank (we do not of course mean 

 higher in the same line, as we say higher genus in a family, or higher order 

 in a class) are such solely from their being more comprehensive, or present through- 

 out a greater range of specieg. 



What is true, therefore, in respect to characters of genera, is likely to be 

 true in respect to characters of higher groups, such as we have been consider- 

 ing in the preceding pages. Believing, then, that a new genus has been es- 

 tablished by the transition of a number of species of a preceding genus in 

 order, without necessary loss of specific characters, I think the same process 

 may have established the suborders and orders in question. That is, that a large 

 number of genera have near the samt time, in past or present geological history, passed 

 into another suborder or order by the assumption or loss of the character or charac- 

 ters of that to which they were transferred, and that without necessary loss of their 

 generic characters . 



I will cite a probable case of this kind, the facts of which I have already 

 adduced. 



It has already been shown that the genera of six of the families of the Ba- 

 trachia Anura form series characterized by the successive stages of ossifica- 

 tion of the skull, terminating in a dermoossified condition, with over-roofed 

 temporal foss^. That in nearly all the other families similar relations be- 

 tween genera exist, but are nowhere carried so far. The character attained 

 by all the first series is now only generic, but should all the genera of each of 

 the six families assume this character in time, as is necessary in accordance 

 with a development hypothesis, it would at once possess a new and higher 

 importance, and would become ordinal or otherwise superior. It would define 

 a series homologous with all those types which had not attained it. This 

 character of the over-roofing of the temporal foss* has actually attained a 

 family significance among the Testudinata, — e. g., as defining the marine 

 turtles ; and similar characters are found by Owen to characterize the 

 Labyrinthodontian order of Batrachia.* 



Agassiz has pointed out a similar and more extended case, in the Hetero- 

 cercal and Homocercal ganoids. Had we not so many of the closest approxi- 

 mations between members of these groups, they would stand in the systems 



♦The roof here alluded to by Owen includes some two distinct bones not known in the 

 arch of the Anura, and therefore different. It is, however, enough to know that this 

 structure is serially associated with its absence and rudimental appearance in the tailed 

 Batraoiiia of the present day, to make the comparison apposite. 



1868.] 



