On the Distribution of Marine Animals. 117 



adjoin the counter septum, — either on the convex or concave 

 side, and whether it be long or short. 



We thus arrive at a more complete definition of the primary 

 counter septum: it is that (I) in relation to which the con- 

 tiguous septa are arranged in a parallel direction, and (2) which 

 has adjoining primary interseptal chambers, containing no 

 secondary principal septa. 



Duerden is not correct, in stating that the main septum and 

 the counter septum lie respectively on the convex and on the 

 concave side of the coral independently of the arrangement 

 of the contiguous septa. This very arrangement has been 

 regarded by palaeontologists as characteristic of the primary 

 septa, and, as will be seen from the above, it is more 

 permanent than has hitherto been known. 



XIII. — On the Distribution of Marine Animals*. 

 By Prof. MTntosh, M.D., LL.D., F.R.S., &c. 



The distribution of land-animals is a subject which has 

 always been fraught with deep interest to naturalists — more 

 especially as certain regions are characterized by the forms 

 inhabiting them. Tims it would be anomalous to find, for 

 instance, a marsupial in Africa, an armadillo or a sloth 

 (Bradypus) in Asia, or a stag in Australia. The chief 

 barriers, moreover, to the general distribution of such forms 

 have been mountain-chains, deep tracts of the sea, barren 

 regions such a., the great deserts, and the vicissitudes of 

 temperature. Yet certain aerial forms, such as the bats, are 

 more or less cosmopolitan, and the shrews, the pigs, and the 

 mice are almost so. In weighing the statement, however, 

 that the distribution of certain of these forms, such as the 

 pigs, has been extended by their swimming powers across 

 arms of the sea, it has to be borne in mind that even marine 

 animals do not always avail themselves of the lines of migra- 

 tion at their disposal. 



As three fourths of the surface of the globe are composed of 

 water — for the most part continuous throughout — a vast field 

 exists for the distribution, under natural conditions, of its 

 inhabitants, from mammals to Protozoa. Pelagic types may 

 thus range from pole to pole and from the eastern shore of 

 the Isthmus of Panama round the world to the western. 



* Kot33 of an Introductory Lecture, 16th October, 1903. 



