EVOLUTION. 319 



It is assumed that Lingulella-lihe forms developed from primitive Oholus in early Cambrian 

 time, since the genus is first kno\vn from the lower portion of the Lower Cambrian and is known 

 to occur throughout the Cambrian and into the Ordovician. The subgenus Lingulepis appears 

 in ]\Iiddle Cambrian time and continues on into the Lower Ordovician. Little is known of 

 Delgadella of the Lower Cambrian, but from its form it is referred to the Lingulella phyllum. 

 Lingula is probably descendant from Lingulella in late Cambrian time. Lingulella davisi (PI. 

 XXXI, figs. 6e, 6f) is quite similar to Lingula, as are Lingulella lens, L. concinna (PL XXXIII), 

 and other Upper Cambrian species. The subgenus Leptemiolon branches off from Lingulella 

 in late Upper Cambrian time. 



Matthew [1902b, p. 98] has given a diagram shomng "Conjectural Lines of Descent of 

 the Canadian Oboli with Reference to the Oldest Known Atremata." This diagram and the 

 discussion following it are based upon Matthew's observations in the Cambrian section of New 

 Brunswick and Nova Scotia and his interpretation of the genera and species. The greater 

 opportunity offered by larger collections from a much wider field has so changed the data upon 

 which his conclusions were based that I shall not enter into a discussion of them. 



Kutorgina (PL V) is assumed to have branched off from the Micromitra radicle in pre- 

 Cambrian time, and from its line of descent or radicle Schucliertina (PL LI, figs. 6, 6a-m) is 

 taken off. Both of these genera are placed on the border line between the Atremata and 

 Protremata. 



NEOTREMATA. 



The progressive increase in the differences in form and the relations of the two valves in 

 the genera of the Neotremata is best seen by a glance at the diagram on page 317. In tliis an 

 O&oZeZZa-hke type is descendant from a simple form like the Rustella radicle and from this radicle 

 the Acrotretidse diverges. Acrotliele, mth its great vertical range from the Lower Cambrian to 

 the Ordovician and its wide geograpliic distribution, is the oldest and simplest form; Acrotreta 

 is derived from Acroihele, and is first known toward the close of the Lower Cambrian and 

 continues through to the Ordovician; from the Acrotreta branch AcrotJiyra and Discinopsis 

 diverged in Middle Cambrian time. Linnarssonella is assumed to have branched off from the 

 Acrotreta radicle, and it is knowTi to have continued from the central Middle Cambrian to later 

 Upper Cambrian time. Another branch from Acrotreta leads to Oriiculoidea in the central 

 Middle Cambrian, wliich continues on into the Ordovician. A shell doubtfully identified as 

 Philhedra occurs in the Middle Cambrian. If correctly identified it represents the Craniidffi in 

 the Cambrian fauna. 



Botsfordia is an offshoot from Oiolella in the central Lower Cambrian, and I have assumed 

 it to be the progenitor of Scliizopliolis of the Middle Cambrian. Quehecia of the middle Lower 

 Cambrian is tentatively shown to be derived from the Oholella radicle. 



Another branch is Trematoholus , which is assumed to be the direct progenitor of the Siphon- 



otretidse as an offshoot of the Oholella primary stock. Yorlcia is the first known branch from 



Trematoholus in Lower Cambrian time. Dearhornia, as laiown, is limited to the upper portion 



of the Middle Cambrian. 



PROTREMATA. 



The ancestral types of the true Protremata are, so far as at present known, Nisusia and 

 Billingsella. These appear to have come from the radicle leading from Micromitra to Kutorgina 

 of the Lower Cambrian. Kutorgina is placed on the border line between the Atremata and 

 Protremata without direct descendants. 



ScJiuchertina (PL LI, figs. 6, 6a-m) has a calcareous shell, smooth outer surface, poorly 

 defined cardinal areas, open delthyrium, and large muscle areas on the ventral valve. It sug- 

 gests one of the transition forms between Kutorgina and Nisusia, but as its stratigraphic posi- 

 tion is in the central Middle Cambrian, far above and much later in time than the early forms 

 of the lines of descent of the Protremata, it is placed on the border line without descendants. 



