327 



Raymond (46) has pointed out a number of interesting cases of reca- 

 pitulation. The very common and well-known Devonian Spirifer, 8. mu- 

 cronatus, has the cardinal extremities in the adult very acute (mucro- 

 uate), sometimes, indeed, drawn out into needle-like points; while the 

 number of plications may be thirty or more. In the neanic stage these 

 transversely elongated spirifers pass through forms corresponding to the 

 adults of certain Niagara species. The adult of S. crispus, corresponds 

 very closely in shape, number of plications, and shell index with these 

 young specimens of S. mucronatus. 



Shinier and Grabau (51) have shown that in the upper part of the 

 Hamilton series of Thedford, Ontario, there occurs a variety of Spirifer 

 mucronatus, which though not mucronate at all in the adult, is "extremely 

 mucronate" in the neanic stage. At this stage also there is evidence of 

 the median plication of the sinus, another charactertistic of the adult of 

 the normal 8. mucronatus. In the adult of the Thedford variety this 

 median plication has disappeared. The geological and morphological evi- 

 dence of the derivation of this form of 8. mucronatus is complete. 



I have pointed out an exactly similar case in the variety senex of 

 PlatystropMa acutilirata (16). This variety occurs in the upper part 

 of the Whitewater division of the Richmond series of Indiana and Ohio. 

 PlatystropMa acutilirata, as is well known, is very mucronate in the 

 adult, resembling in its general outline, Spirifer mucronatus. It was in 

 fact at first referred to the genus Delthyris (Spirifer). The normal form 

 is shown by an unusually closely graded series of intermediate forms to 

 be descended from P. laticosta, and it repeats the adult characters of the 

 latter very faithfully in its late neanic stage, becoming always more mu- 

 cronate as development proceeds. The upper Whitewater form, var. 

 senex, frequently has entirely lost, in the adult stages, the acute angula- 

 tion of the cardinal extremities, so that the lateral and cardinal edges 

 make a right, or nearly a right angle. In the young (neanic) stages of 

 P. senex, however, the shell is decidedly mucronate, so that these young 

 shells exactly resemble the normal PlatystropMa acutilirata of the lower 

 Whitewater and Liberty formations. P. senex, it may be remarked, is a 

 well denned form, and its derivation from P. acutilirata is beyond ques- 

 tion, since it is connected with the latter by every gradation. 



Another interesting case of recapitulation among the brachiopods has 

 been worked out with great care by Mr. F. C. Greene (27). In this case 

 also no pains was spared to ascertain the relationships of the various 



