248 PROCEEDINGS OF THE ACADEMY OP 



Now, individuals of tlie genus Cervus of the second year do not belong to 

 Subulo, because they have not as yet their mature dentition. Ruga, however, 

 is identical with those Cervi whose dentition is complete before they gain the 

 antlers of the fifth year. When the first trace of a snag appears on one beam 

 of Cariacus virginianus, the dentition includes the full number, but there 



remain _ milk molars much worn and ready to be shed. Perhaps the snag is 



3 



developed before these are displaced. If so, the Cariacus is never a Subulo, 

 but there can be little doubt that the young Blastocerus belongs to that genus 

 before its adult characters appear. 



7. Leidy states* that certain Perissodactyle remains containing a foot of a 

 horse, contained the teeth of a genus, Merychippus, which has the per- 

 manent teeth of Equus, and the deciduous dentition of Anchitherium. He 

 observes " the deciduous and permanent dentitions of both these genera are 

 alike, therefore the new genus is in early life an Anchitherum, and later in life 

 a true horse." This is therefore a case of exact parallelism, always providing 

 that the Merychippus has not added to its immature Equine chai-acters, others 

 in other parts of the body, which invalidate the indentity. In the latter case, 

 it will still be an interesting example of the " inexact parallelism." 



8. It is well known that the Cephalopoda form a number of series of re- 

 markable regularity, the advance being in the first place in the complication 

 of the folds of the external margins of the septa, and in the second place in the 

 degree of involution of one or both extremities of, the shell to the spii'al ; third, 

 in the position of the siphon. 



Alpheus Hyatt, in an important essay on this subject,! points out that the 

 less complex forms are in many cases identical with the undeveloped condi- 

 tions of the more complex. He says : " There is a direct connection between 

 the position of a shell in the completed cycle of the life of this order, and its 

 own developement. Those shells occupying the extremes of the cycle " (in 

 time), " the polar forms, being more embryonic than the intermediate forms. { 

 The first epoch of the order is especially the era of rounded, and, in the ma- 

 jority of the species, of unornamented shells with simple septa ; the second is 

 the era of ornamentation, and the septa are steadily complicating ; in the third 

 the complication of the septa, the ornamentation, and the number of species, 

 about twice that of any other epoch, all combine to make it the zenith of de- 

 velopement in the order; the fourth is distinguishable from all the preceding 

 as the era of retrogression in the form, and jiartially in the septa. 



" The four-periods of the individual are similarly arranged, and have com- 

 [larable characteristics. As has been previously stated, the first is rounded 

 :ind smooth, with simple septa ; the second tuberculated, and the septa more 

 complicated; the third was the only one in which the septa, form and orna- 

 mentation simultaneously attained the climax of individual complication ; the 

 fourth, when amounting to anything more important than the loss of a few 

 ornaments, was marked by a retrogression of the whorl to a more tabular as- 

 pect, and by the partial degradation of the septa." 



I will here quote an entirely antagonistic statement of Bronn's,| as follows : 

 In the developmeait of Lamellibranchiate molluscs "it is not possible to esti- 

 mate the successional changes of one genus by those of another, though nearly 

 related ; so diverse are the most significant relations in the manner of progress 



* Proceed. Acad. Nat. Sci., 1858, p. 7. 



f Memoirs Boston Soc. N. Hist, 1806, 193. 



1 He adds here: "Although in regard to geological sequence and structiTral position 

 one of the extremes must be of higher geological rank." The "/uV/Ztesr' extrenie will be 

 of higher geologicai rank according to the complexity of structure and length of develop- 

 mental scale, whether it come at the middle or end of the history of the class in time. If, 

 us has V)een the case so far as known, a decline has terminated the history of a class, its 

 later forms are zoologically lower than its older ones. Hence the adjective /tiV/ft is only 

 appropriate to types of the latter kind, when used as synonymous with extreme. 



<! Classen u. Ordnungen des Thierreichs, iii, 445. 



[Oct. 



