392 ROBERT TRACY JACKSON ON THE 



Plicatula is an iiiidoiibted ally of Spoiiclylus, as usually considered. As we have 

 shown, the shell of Plicatula becomes attached at the close of the prodissoconch stage, 

 and at once assumes the ostrean form, thus eradicating features of the nepionic stage 

 which (as in Spondyhis and Ilinnites) might render that stage compai-al)le to Pecten. 

 Plicatula is considered the farthest removed from Pecten of its group, for in it the ostrean 

 form is earliest acquired and most completely emphasized in the shell. (Further descrip- 

 tion of Plicatula, see p. 352.) 



The table on the preceding page represents graphically the genetic relations expressed 

 at various places in this paper and especially in the pi'esent section. AVhen genera are 

 considered as derived from other genera and in the ascendent line of the group, they are 

 placed directly over one another, as in the case of Gryphsea and Ostrea. When a genus 

 is derived from another, but is not considered as in the ascendent line of the series then 

 it is jjlaced as a side branch, as Alectryonia from Ostrea. Bi'oken lines are used where 

 the affinities expressed appear to be open to doubt. With each genus is expressed in 

 brackets its earliest geological appearance, the data for these statements being for the 

 most gathered from Zittel's Palaeontology. The following abbreviations are used: Low. 

 Sil., Lower Sihu-ian; Up. Sil., Upper Silurian; Dev., Devonian; Carb., Carboniferous; 

 Perm., Permian; Trias., Triassic; Jura., Jurassic; Cret., Cretaceous; Tert., Tertiary; 

 Rec, Recent. 



ERRATA. 



Page 281, note, the prodissoconch is described in a few genera not incUided in tliis list. For such additions sec sections 

 VIII, X and XIV. 



Page 291, paragraph .5, line 5, for Apysia, read Aplysia. 



Page 291, paragraph 6, attention sliould have been called to Dr. W. Branco's monograph: Beitrage zur Eatwickelungs- 

 geschichte der fossilen Cephalopoden. Pala;ontographica, B. 27, 1880-81. 



In section IV, at several points the oyster is spoken of as showing concentration of development, or as a concentrated 

 type. In these cases for concentration, or concentrated, read acceleration, or accelerated, as that woj'd is more in accord with 

 the principle involved. 



Page 314, line 14, for overlying, read underlying. 



Page 330, line 5, for Leioptera, read Leiopteria. 



Page 333, line 8, from bottom of page, for shortening its length. It, read then shortening its length, it. 



Page 334, paragraph 3, line 13, for exists, read were existent. 



Page 335, paragraph 3, line 10, for at that time, read therefore. 



Page 336, paragrapli 3, line 8, for specimen if kept, read specimen kept. 



Page 33G, paragraph 3, line 12, for condition, read stage. 



Page 337, paragraph 2, line 4, for If one break the byssus loose it, read If the scallop breaks loose, the byssus. 



Page 338, paragraph 2, line 0, for over, read at. 



Page 345, line G, for Actinoptera, read Actinopteria. 



Page 361, description of cut, for eo, read 30. 



Page 366, line 3, for adults, read young shells. 



Page 370, paragraph 3, line 12, for F///read VIL 



