DE VEL OP ME NT. 5 9 



terstitial tubes become covered with " a thin smooth calcare- 

 ous membrane," leaving the larger tubes open, and causing 

 their mouths to assume a circular or oval shape, and to project 

 above the general surface. Lastly, the colony is»said to change 

 into a Mojiticztlipora by the development of regular " monti- 

 cules," which are " arranged in quincunx, and formed at the 

 points where seven or eight large cells are clustered." 



In the preceding I have endeavoured to give a faithful 

 account of the views which Dr Lindstrom has published as 

 to the development of the Monticidiporcc, and upon which 

 he, in large part, bases his view that the fossils of this genus 

 are really Polyzoa. Not having had the opportunity of per- 

 sonally examining the specimens upon which his views are 

 based, it would be presumption on my part were I to impugn 

 the accuracy of the description which he has given of the 

 phenomena which he has observed— the more so as his justly 

 deserved reputation is a guarantee that he has not arrived 

 at the conclusions in question without sufficient consideration. 

 At the same time, I regret to find myself in the meanwhile 

 unable to accept these conclusions ; and though I cannot 

 here enter into the subject at length, I may just briefly indi- 

 cate the principal reasons which lead me to dissent from the 

 views of such a high authority upon this and kindred ques- 

 tions. In the first place, then, it is clear that the study of the 

 development of a fossil organism is attended with difficulties 

 much more serious than those which are incidental to a similar 

 investigation in the case of a living animal ; since in the latter 

 it is generally possible to trace the actual transition from one 

 stage of growth to another. This, by the nature of the case, 

 is rarely — one might almost say never — possible in the case 

 of a fossil. It is true that in the passage of what he has 

 termed the " Fistulipora stage " to the " Thecostegites stage," 

 Dr Lindstrom states that he has actually seen the same speci- 

 men exhibiting the characters of both stages in different parts 

 of its skeleton. Still the passage between the two stages just 

 referred to is a comparatively small step to make, and it does 



