19b 



THE AMEBIC AN NATURALIST [Vol. XLVIJ 



arcuate, loose-coiled, then close-coiled stages directly 

 comparable to the adults of Paleozoic cyrtoceran, gyro- 

 ceran and nautiloid representatives of its own group. 

 As shown by J. Perrin Smith, 4 the highly evolved Cre- 

 taceous Placenticeras pacificum which in the adult has 

 complex sutures, in the development of these parts 

 passes through simpler stages which are comparable to 

 the adult structures of nautiloid, goniatitic and glyphio- 

 ceran forms, followed by stages in which the septa are 

 comparable to those of early Ammonites, before it as- 

 sumes its adult generic features. The recent Pecten has 

 strongly marked ears, but the young shell is strikingly 

 different, first having a rounded nuculoid form, followed 

 successively by Rhomb opteria, Pterinopecten and Avi- 

 culopecten stages before its adult character is attained. 

 In Echini the recent Goniocidaris and other genera, 

 both recent and fossil, have two or more columns of 

 plates in each interambulacral area, but in the young 

 they pass through a stage in which there is a single plate 

 at the ventral border of the interambulacra, which is 

 comparable as a stage in development to the adult of the 

 Ordovician Bothriocidaris which retains a single column 

 of plates in each interambulacral area in the adult. The 

 Lower Carboniferous echinoid Oligoporus which in each 

 ambulacrum has four columns of plates with, in addition, 

 scattered isolated plates, passes through stages with 

 primary plates only, as in Palceechinus, then primary and 

 occluded plates as in Maccoya, followed by four columns, 

 without isolated plates, as in Lovenechinus, before at- 

 taining its generic character. In Brachiopoda, as shown 

 abundantly by Beecher and others, stages in develop- 

 ment are shown in the exterior and interior of the shell 

 and the brachial supports which can be closely correlated 

 with adult characters of more primitive representatives 

 in the group. 



While stages in development from the young to the 

 adult are typically all progressive, in senescence, the 



4 J. P. Smith, "The Development and Phylogeny of Placenticeras," 

 Troc. California Acad. Sci.. Ser. 3. Geology. Vol. 1. No. 7, 1900. 



