. I I'll \l <>I-,,I.\ 



"Ml,... t .. blhnr.1,1,., ! 



The Mirf.K ( i- ornamented l.y coarse ami fine vertical, elevated lines, reticulated 

 by extremely fine hon/unta! linr<. Toward the apex, over the smooth i>ortion of 

 tin- >hell. the vertical lines occur in two >imple series; where the shell has a diame- 

 ter of 7 nun. there are twelve lines of the first order, between each two lying one of 

 secondary series. As growth advances these lines rapidly multiply by intercala- 

 tion, an. I the alternation in the size of the stria) becomes decidedly less pronounced. 

 Oer the annulated and later portions of the shell the ornamentation becomes 

 proi>ortionally very much finer but the regularly alternating size of the lines is 

 maintained throughout. The horizontal striiu are exceedingly fine and often not 

 retained. Where crossing the other series they are usually elevated into slight 

 nodes or projections. 



Sipho small and nearly central.* The septa are rather shallow and the sutures 

 regularly transverse and without undulations. They bear no definite relation to the 

 annulations. Over the early, smooth portion of the shell they appear to be relatively 

 distant on account of the narrowness of the shell, there being seven air-chambers in 

 a length of 17 mm., in another specimen five in a length of 12 mm. They do not 

 greatly vary in depth with the increase in the diameter of the shell. The sutures 

 being usually t ran Averse, cross the more or less oblique annulations and constrictions, 

 variously transecting, or at times lying wholly within a given furrow. 



The original description of this fossil was based upon specimens showing only 

 the adult characters of the species. The existence of specimens in the material in 

 hand, showing in a single example the gradual change from a smooth to an annu- 

 lated shell, brings out an interesting fact in regard to the morphic variation through 

 which other annulated species are known to pass. It has, for example, been shown by 

 Hall' that the embryonic tip of the shell of Orthoctras crotalum, an annulated 

 Devonian species, is smooth, and also that the vertical stria; are well developed 

 much before the appearance of the aim illations. In that species, however, the 

 smooth portion of the shell is very short and greatly abbreviated in comparison with 

 that <>f n. liilinrtilmn. The passage of the shell of 0. crotulum through the smooth 

 stage is highly accelerated, while its longer duration in O. liHim-utmii more forcibly 

 suggests the pbyletic as well as individual relation of the non-annulated to the 

 annulated forms of this genu- 



It is, however, to be observed that the degree to which the apical smooth shell 

 of 0. bilineatum is retained is in a certain sense an individual peculiarity. Some 

 specimens develop the annulations much earlier than others, and those which retain 

 the smooth shell to a considerably later period preserve for a longer period an 



infantile character. 



York. ol. T. pi U. pi. cam. . 11. 



