INVEKTEBBATA, ORYPTOGAMIA, MICROSCOPY, ETC. 763 



bable, and consequently is true unless the contrary be proved. I only 

 wisli to note that this is a reversal of all the laws of evidence in any 

 case of fact whatever, and to add that, so far as I have had the oppor- 

 tunity of observation, no proof has reached me of progressive, perma- 

 nent, and essential change in molluscan development." 



Antiquity of certain Subordinate Types of Fresh-water and 

 Land MoUusca.* — Mr. White points out that of the minor groups 

 into which some of the " comprehensive " genera of these forms have 

 been divided, a large number had their origin in periods which were 

 at least as early as the closing epochs of the cretaceous or of the 

 eocene periods. After a technical demonstration of these points, the 

 author, on reviewing the collections, finds that there are in it so 

 many "familiar forms" that it seems difficult to imagine that a 

 large number " were living contemporaneously with the last of the 

 Dinosaurs." The changes these Mollusca have endured seem to be 

 very remarkable ; there was a " gradual desiccation of the regions 

 formerly occupied by the great inland lakes," " the elevation of the 

 whole Eocky Mountain system, and the establishment of the present 

 great interior river-systems." Although some forms have disappeared, 

 " the lines of descent of the numerous types which have reached us 

 unbroken seem to be almost parallel," and the author comes to the 

 conclusion that in some degree at any rate these types have had a 

 " saltatory " origin, although he allows that the mode must always 

 remain obscure. 



Development of the Digestive Tract in the Mollusca. t — From 

 an abstract of the researches of Dr. W. K. Brooks we learn that he has 

 come to certain definite conclusions, of which the following note gives 

 an account of some of the most important : — 



(1) The polar globules mark the principal axis of the egg. 



(2) When there are four equal sjjherules in the egg, the proto- 

 plasm of each is segregated ; that which will give rise to the ectoderm 

 occupies the formative end and is quite transparent. 



(3) These formative ends separate as four micromeres. 



(4) By their division, and by the separation of other cells from 

 the formative end of the macromercs, an ectoderm is formed, which 

 entirely covers the four macromercs except at the blastopore. 



(5) These macromercs now become fused, and part becomes sepa- 

 rated to form the endodermal layer of cells. 



(6) The remainder divides into a largo number of cells, which 

 occupy an intermediate position. 



(7) These are not food-yolk, but continue to grow. 



(8) The ectodermal cells about the blastopore become converted 

 into the shell-area. 



(9) The mouth is an independent invagination of the ectoderm. 



(10) Which does not become connected with the digestive tract 

 until after the closure of the blasto])ore. 



(11) The stomachal appears to be the same as the primitive cavity. 



* ' Aiuer. Journ. Sci.,' xx. (1880) p. 44. 



t ' True. JJostou Sue. Nat. Hist.,' xxx. (18S0) p. 325. 



