PLASTICITY AND PERMANENCY OF CHARACTERS. 3O/ 



breeding and limitation of range of variability by hereditary 

 transmission. 



E. Surface Markings. The surface of Spirifers, when 

 well preserved, are almost always covered with fine longitudi- 

 nal or radiating lines, or these interrupted by concentric lines 

 or irregular papillary elevations (see Figs. 8395). Judging 

 from the structure of living Brachiopods, these are associated 

 with certain setose prolongations of the edge of the mantle, 

 or bristles, and their appearance in the structure of the shell 

 surface may be due to a growing around the individual bristles 

 of the extreme edge of the shell, so that the striae are of im- 

 portance. In some (Spirifer fimbriatus, lineatus, etc.) the 

 size is large enough to show the openings, which are quite 

 complicated and resemble the opening of a double-barrelled 

 gun. The modification of this feature is by increase or de- 

 crease in size of the striae, by interruption regularly or irregu- 

 larly. When interrupted regularly, it appears to be by a 

 periodical stoppage of growth, and thickening of the shell 

 lamellae, forming on the surface imbricated structure, the striae 

 starting anew at each successive imbrication. The fact that 

 they are surface striae is also so accounted for, the deposit of 

 shell filling up all the under side of the striae. In Sp. plica- 

 tella (Figs. 84, 85) the whole surface is uniformly covered 

 with these continuous radiating striae. In 5. crispus (Figs. 

 88, 92) the surface is interrupted by imbrications, and is cov- 

 ered by regular rows of the interrupted lines. 



A comparison of series of successive species, which by 

 their general combination of characters may be supposed to 

 have been in direct line of genetic succession, shows a gradual 

 diminishing in size of the striae, and in case of the continua- 

 tion and increase of imbrications there results an entire 

 absence of the striae at least they fail to be discernible on 

 specimens. 



The particular size and form of these striae seem to be a 

 very delicate means of tracing the lines of hereditary succes- 

 sion, or what we may suppose to be such lines; for species 

 which in other respects are very much alike can be easily dis- 

 tinguished by this character if the surface be well preserved. 

 The modification of this character appears to be in two 



