312 CMIBRIAN BRAGHIOPODA. 



examples of Finkelnhurgia (PL XCIII, figs. Id, If, 2, and 2b) there are indications of large main 

 trunks, and in Orusia (PL XCVIII, figs, le and Ip) narrow trunks are seen. Only one among 

 many hundreds of specimens of Eooriliis (PL XCI, fig. 3b) shows traces of the main trunks. 



Among the Syntrophiidse, HueneUa abnormis (PL CIII, figs. 2b and 2h) shows that the main 

 trunks were of large size. I have not seen any interiors of the shells of the Strophomenidse from 

 the Cambrian system. 



This brief review proves that the simple primitive type of the circulatory system, as developed 



in the main vascular canals, their laterals, and the peripheral canals of the mantle, was fully 



developed in Lower Cambrian time, and that this type persisted throughout Cambrian time. 



The modified form of the canal system of the Craniidfe has not been observed, nor has the 



complex canal system of the Ordovician and later orthoids [Hall and Clarke, 1892c, Pis. Va, Vc, 



VI, and VIA]. 



MUSCLE SCARS. 



It is only within a few years that material has been available from Cambrian rocks that 



could furnish data for a description of the muscle scars and by them an outline of the muscular 



system of the Cambrian brachiopods. 



ATREMATA. 



In the Atremata this has been most thoroughly worked out for Oholus by Mickwitz (1896), 

 and an outline may be found in this monograph under the description of the genus Oiolus. The 

 terminology used in this monograph is mentioned on page 292. 



In the most primitive form of brachiopod known to me, RusteUa edsoni, the muscle scars of 

 the dorsal valve, so far as known, indicate that the system of scars was essentially the same as 

 in Oholus. In the dorsal valve the central scars (PL I, fig. le) are preserved; also the anterior 

 portion of the outhne of the parietal band, which indicates the position of the anterolateral 

 scars. Although we have a number of good natural casts of the interior of the ventral valve, 

 no traces of vascular markings or muscle scars have been observed. So far as known none of 

 the many hundreds of specimens of various species of Micromitra have preserved any definite 

 outlines of the muscle scars, and the same is true of Mickwitzia and Helmersenia. 



Oholus and Lingulella and their subgenera all have the same general grouping of the 

 muscle scars as described under Oholus. For Oholus these are well shown in 0. apollinis (PL 

 VII) ; for Lingulella by L. acutangula (PL XVII) ; for the subgenus Westonia by Oholus (Westonia) 

 escasoni (PL XLIX, figs, la, laa, Ic, and Id). The same general arrangement of the muscle 

 scars, so far as known, is found mBicia (PL L), ETkania (PL LI), and Oholus (Fordinia) (PL 

 LXIII, figs. 10a and 10c) . In Dicellomus the central and the outside and middle lateral scars are 

 located in front of the visceral area, as in Oholus, but the anterolateral and transmedian scars 

 are combined in one large cardinal scar (PL LIII, fig. Ic at cl), as in Acrotreta (PL LXIV, figs. 

 2c and 2d) ; in the dorsal valve the central and anterolateral scars are as in Oholus, while the 

 transmedian, outside laterals, and middle laterals appear to be grouped in one large cardinal scar 

 (PL LIII, fig. li at cl). The combining of the muscle scars of the posterolateral region of the 

 valves in one large scar in this genus of the Atremata foreshadows the typical large cardinal 

 scar so characteristic of the Neotremata. 



NEOTREMATA. 



In Oholella (Pis. LIV and LV) the arrangement of the muscle scars, so far as Imown, is that 

 of Oholus, but with a tendency on the part of the posterolateral scars to close in toward each 

 other; this tendency is carried into effect in Botsjordia (PL LIX, figs. Ik and In; PL LVII, 

 fig. 4k; and PL LXII, figs. 5g and 5i). In the dorsal valve of Botsfordia pulcJira (PL LXII, 

 fig. 5h) the transmedian and middle lateral scars appear to be distinct, but in figure 5i and in 

 all ventral valves, figures 5g and 5j , a single large scar appears to be present. luLinnarssonella, 

 which I have placed as the descendant from the Acrotreta stock (schematic diagram, p. 317), 

 the posterolateral scars are all grouped in both valves in a large cardinal muscle scar (PL 

 LXXIX, figs. Ik-p). The muscle scars of the central portion of the valves remain essentially 

 as in Oholus. 



