NO. 1846. ON CERTAIN ELEVTHEROZOIC PELMATOZOA—KIRK. 89 



Mr. Springer, on page 31, considers that the change from one type 

 to the other might have taken place in one of three ways : 



1 . The resorption of the inf rabasals in Form D. 



2. The intercahition of inf rabasals in Form M. 



3. The coalescence of the inf rabasals with the centrale. 

 Although I do not beUeve that the change was brought about by 



the second process, and can accept the first hypothesis only as a 

 partial explanation, either is a possible explanation for the facts as 

 we know them. Therefore it is best to consider the arguments for 

 and against both before taking up the discussion of the third, which, 

 as before stated, I beheve to be essentially the correct interpretation. 



The argimient brought against the first two hypotheses by Mr. 

 Springer is that of the orientation of the centrale, wliich he considers 

 an insuperable difficulty. As shown above, the centrale is dependent 

 as regards its form on the plates surrounding it. To show what a 

 simple matter it would be to change the orientation of the centrale by 

 the intercalation of inf rabasals consider ffi-st figure 11, Plate 9, a 

 typical Form M, with an unusually large centrale. Here the centrale 

 is pentagonal and radial. Xow, at the apices of tliis pentagon sup- 

 pose we introduce five minute infrabasals. To allow the introduction 

 of these infrabasals, however small, there is a synchronous resorption 

 of each apex, wliich becomes more marked with the increase in the 

 size of the infrabasals. We now have a decagon, such as is beauti- 

 fully shown in figure 3, Plate 9. If the infrabasals continue to grow 

 there will be a corresponding resorption of the centrale along the sur- 

 faces in contact with the infrabasals. It is obvious that soon the 

 infrabasals wiU meet laterally, and the inclosed centrale instead of 

 being a radial pentagon or decagon will be transformed into an inter- 

 radial pentagon, resulting in a form hke figure 2, Plate 9. So far as 

 orientation is concerned this hypothesis is perfectly sound. If we 

 reverse the series outhned above we can see how the form of the cen- 

 trale can be altered from interradial to radial, if we wish to estabUsh 

 the vahdity of the infrabasal resorption theory. Here, starting with 

 figure 2, Plate 9, let us consider that the centrale gradually enlarges. 

 As it increases in size there is a concomitant resorption of the apical 

 portions of the infrabasals. Before long we reach the decagon stage, 

 represented by figure 3, Plate 9. If this process be continued but a 

 short time longer these minute remnants of the infrabasals would dis- 

 appear, their places being taken by the enlarged centrale. A very 

 sHght straightening of the Hues of contact between the centrale and 

 basals would give us a typical Form M, with a radial centrale. That 

 such resorption and mutual readjustment of plates is quite possible 

 will be sho\vn hereafter at some length. 



As shown above. Form D could be evolved from Form M, providing 

 the requisite infrabasals were suppUed. But where could these plates 



