290 DR. A. E. TBUEMAN OX THE [vol. lxXlV, 



have been described as Androgynoceras sipliunculare and A. in- 

 tegricostatum, which occur in the gagateus band (oxynotuni zone) 

 of Robin Hood's Bay. These ammonites are smooth until a 

 diameter of 3 mm., and their sutures are simpler than those of the 

 later forms at corresponding diameters. 1 



The species figured by Reynes (6, pi. 1, figs. 26-32, pi. xlv, 

 figs. 47-49) from the raricostatum zone (A.sirius and A. vesta) 

 are very little more advanced than these forms. A. circumdatus 

 Reynes (6, pi. xxviii, figs. 19-22) described from the buclclandi 

 zone and placed by Mr. Buckman on the line of descent of the 

 capriconis, 2 is probably a member of another series, and cannot be 

 considered as the early ancestor of the Liparoceratidse ; but it may 

 represent an offshoot from their ancestral stock. 



It is probable, however, that the Capricorn forms possessed their 

 essential characters . at a comparatively early date ; thereafter 

 evolution proceeded slowly for some time, and consisted solely in 

 the acceleration of the Capricorn ornament and the increase of 

 complication of the sutures. The development of bituberculation 

 and the sphaerocone form in each series was comparatively sudden ; 

 this period of rapid evolution appears to have frequently followed 

 the arrival of a series in this area. 



The Pre-Capricorn Ancestor. 



It has been shown that the inner whorls of the Liparoceratiche 

 are invariably smooth and depressed, with a relatively- narrow 

 Umbilicus ; the vouno- of manv ammonites of various families have 

 a similar form. Prof. J. Perrin Smith has suggested 3 that the 

 Glyph ioceras-\ike young may be explained in two ways : — 



(1) They may indicate that all ammonites are descended from a CrlypJiio- 



ceras-like ancestor. 



(2) This form of shell may be coenogenetic and not palingenetic ; that is, 



it may be acquired owing to its suitability during the early life of 

 all ammonites, and may have no reference to the form of the 

 ancestor. 



In support of the latter view it may be mentioned that the 

 protoconch of every ammonite has a depressed form of aperture ; 

 it is, therefore, probable that the succeeding whorls will gradually 

 become more elevated until they attain the adult shape. That is 

 to say, the depressed inner whorls may be a result of the form of 

 the protoconch. and may be independent of the form of the 

 ancestor. 



1 Thus the suture of A. sipliunculare (Whitby Museum, No. 485) at a dia- 

 meter of 20 mm. has an external saddle divided simply into three terminal 

 folioles, and a first lateral lobe with three small lobules, a stage which is 

 attained by the suture of the capricorns described previously at a much 

 smaller diameter (figs. 2e, 9 d, 11 g, pp. 266, 281, 284). 



2 S. S. Buckman, Q.J. G. S. vol. liv (1898) table ii, facing p. 451. 



3 ' Acceleration of Development in Fossil Cephalopods ' Stanford Junior 

 University Publications, 1914. 



