460 CAMBEIAN BRACHIOPODA. 



toward the beak. The convexity gives a depth of about 1 mm. to shells 5 mm. in diameter. 

 Surface marked by radiating strise, concentric lines of growth, and fine, more or less undulating 

 transverse stride. When the thin outer layer of the shell is exfoliated, the next layer is marked 

 by rather strong, radiating strise and concentric striss and lines of growth. The inner surface 

 of the shell is strongly pitted, even over the visceral area, and the anterior and lateral margins 

 are marked by the impressions of the fine canals of the vascular system. The character of the 

 transverse lines is fully shown by Plate XLIX, figure If. 



The shell is relatively thick, and formed of a very thin outer layer and several inner layers 

 or lamellse, which are more or less oblique to the outer layer. These layers are well shown by 

 the posterior portion of Plate XLIX, figure la. 



The shell is small, the largest not exceeding 6 mm. in length. The dorsal valve is slightly 

 shorter than the ventral. The relative dimensions of the two valves may be seen by comparing 

 figures 1 and lb of Plate XLIX. 



The details of the area of the ventral valve are not well shown by any specimens in the 

 collection. The pedicle groove appears to have been well marked, and the area to have extended 

 a short distance out on the cardinal slopes. The area of the dorsal valve is larger in proportion 

 than usual when compared with that of the ventral valve. The striae of growth cross it parallel 

 to the anterior margin. 



The interior of the ventral valve is shown by Plate XLIX, figure la, which beautifully 

 preserves the central portion, and by the cast (fig. 1). The heart-shaped pit (x), so character- 

 istic of Oholus, is clearly defined in figure la, where it strongly resembles the same space in 

 Oholus apoUinis Eichwald (PI. VII) . No traces of a median septum have been observed in the 

 ventral valve, but in the dorsal valve the median ridge extends from back of the central muscle 

 scars to the anterior lateral scars. A strong, pestle-shaped depression extends in the ventral 

 valve from the area forward to the anterior margins of the visceral cavity. The visceral area 

 of the dorsal valve is narrow, elongate, and extends forward to the anterior third of the valve. 



Owing to the small size of the shell and the exfoliation along the margins, no one valve 

 shows all of the muscle scars belongmg to it. In Plate XLIX, figure la, the outside lateral, mid- 

 dle lateral, and central scars are beautifully defined, but the transmedian and anterior laterals 

 have not been observed in the ventral valve. Many specimens show the anterior laterals and 

 centrals of the dorsal valve, also the transmedian. The outside and middle laterals appear to 

 be combined in one scar a little anterior to the transmedian. The umbonal and pedicle scars 

 have not been observed. 



Of the markings left on the shell by the vascular system the main or trunk canals are the 

 most prominent. They are, however, rather mdistinct in most examples of the dorsal valve 

 and not specially prominent in the ventral valve. The channels occupied by the fine branches 

 of the exterior lateral canals are well shown by Plate XLIX, figures la and Ic. 



Observations. — I have made a very close examination of Matthew's type material, which 

 he kindly sent to me for examination, but do not find the muscle scars of the ventral valve 

 as shown in his figure Ig [Matthew, 1903, PI. XVI]. On the contrary, as shown in my Plate 

 XLIX, figure la, the outside and middle laterals and central muscle scars are arranged very 

 much as in Ololus. It is exceedingly difficult to determine the exact position and size of the 

 muscle scars, but with a strong magnifying glass and reflected light the duU surface of the 

 point of attachment can often be determined. The pecuUar arrangement of the muscle scars 

 indicated in. Matthew's figure [1903, PI. XVI, fig. Ig] results from examinmg the specimen in 

 a fight which raises the ridges bounding the muscle scars so as to give the effect of transverse 

 lines. I find that the scars are not placed in exactly the same position on the two sides, and 

 that on the left side it is practically impossible to determine the outside laterals. In the study 

 of such minute objects there will always be an opportunity for different interpretation, but 

 with the experience I have had in the study of the Cambrian Brachiopoda, I think the position 

 given the muscle scars in figure la is essentially correct. The characteristic transverse sculp- 

 ture refers the species to Westonia. It is a beautiful shell and one of the most striking forms 

 of this type from the Cambrian rocks. 



