250 



PROCEEDINGS OF THE ACADEMY OF 



Relations between the terms of 

 tlie ditFerent series. Heterolo- 

 gy or Remote Parallelism. 



.S '^ 



'at d 



This is an example of the simplest case of 

 inexact parallelism, as distinguished from the 

 exact parallelism or identity. As the fauna 

 of the present period is but a fragment, so the 

 simple inexact is a more frequent relation 

 than the exact, while the more complex in- 

 exact relation is still more common. The 

 greater the inexactitude, the more frequently 

 do such parallels occur, till we have those 

 of the most remote character, as, for instance, 

 the parallelism between the different stages 

 of the development of the mammal, in the 

 structure of the heart and and origins of the 

 aorta, and the existing classes of vertebrates. 

 The relation of these facts to the origin of 

 genera will be noted hereafter. 



It will be borne in mind that in the Sca- 

 phiopodidfe the generic types are identical 

 for a long portion of their developmental his- 

 tory. 



2. In both Perissodactylous and Artiodac- 

 tylons Mammalia, certain types develoj) their 

 family character of canines at the earliest ap- 

 pearance of dentition, others not till a com- 

 paratively late period of life (Equus), and 

 the extreme genera never produce tliem. 



3. Among Cetaceans the genus Orca main- 

 tains a powerful and permanent series of 

 teeth, which is an important generic charac- 

 ter. In Beluga the series is shed in old age, 

 in Globiocephalus, or the Caing whales, they 

 are shed at middle age, while in the Bala-nidas, 

 of which the absence of teeth is an essential 

 character, these organs are developed and ab- 

 sorbed during fcelal life (Eschricht). Thougji 

 the condition of the teeth is not of system- 

 atic value in the two named intermediate 

 genera, it is the important feature in the his- 

 tory of progress to such value. 



4. Among the tortoises, the Testudinidffi 

 rapidly extend the ribs into a carapace, which 

 fits closely the marginal bones, while equally 

 early in life the elements of the sternum unite 

 together. This is also the case with most Emy- 

 dida' ; among whose genera, however, we find 

 the transitional scale. In Dermatemys and 

 Batagur the carapace is very late in attaining 

 its complete ossification, while the plastron is 

 early finished. In Chelydra, on the other 

 hand, while the carapace is even more slowly 

 developed, the plastron is never free from its 

 larval fontanelles. In the marine turtles 

 neither plastron or carapace is ever com- 

 pleted, while in the Trionychidfe the marginal 

 bones are also entirely undeveloped. 



In order that this last illustration be a true 

 (jne for the theory in question, as apjilied to 

 the families, these developmental characters 



[Oct. 



