296 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvni. 



divide the umbonal cavity into fine chambers. The material available 

 for .study does not show the area in very g-ood preservation; a delthy- 

 rium exists in the area of the ventral valve which is all that can be said. 



This species appears to be a direct descendant of the smooth, convex 

 forms of Si/7itropMa like ^S'. rotundata. 



Formation and locality. — Lower Ordovician. Limestone east side 

 of Gallatin River a))ove Gallatin City, Montana. 



SWANTONIA, ne\^^ genus. 



The description of the type species includes that of the genus, as 

 there is only one other species now referred to the genus and that is 

 very imperfectly known. 



Type. — Camarella antiqua., Billings. 



SWANTONIA ANTIQUATA, Billings. 



CamereUa antigiiata BihhiyiCiS, Pal. Fogs., I, 1861, p. 10, iig. 13. 



Camerella antujuata Billings, Geol. Vermont, II, 1861, p. 949, tig. 353. 



Camarella antiquata Billings, Geol. Canada, 1863, p. 284, fig. 290. 



Camarella antiquata Walcott, Bull. U. S. Geol. Survey, No. 30, 1886, p. 122, 



pi. VII, fig. 8. 

 Camarella. antiquata Walcott, Tenth Ann. Kept. U. S. Geol. Survey, 1891, 



p. 613, pi. Lxxii, fig. 3. 

 Camarella? aidiquata Hall and Clarke, Pal. New York, VIII, 1893, Pt. 2, 



p. 220. 

 Protorliyncha ? antiquata. Schuciiert, Bull. I". S. Geol. Survey, 1897, No. 87, 



p. 334. 



Ventral valve ovate, moderateh" convex ; apex pointed and incurved 

 over the area nearly to the plane of the margins of the valve; surface 

 marked by from 8 to 12 or more rounded ribs that extend back well 

 toward the apex; a slight flattening of the median portion suggests 

 that a shallow median sinus may be found on old shells. The shell 

 illustrated has a length and width of 11 mm. 



A cast of the interior of a ventral valve shows no traces of muscle 

 scars or vascular markings ; two strong teeth are indicated, also a 

 narrow, strong, concave shelf or area ; the area shelf is free from con- 

 tact with the bottom of the valve, a recess or chamber existing 

 beneath it. 



()hserv(ttion.'<. — The area or shelf may ])e considered a short, free 

 spondylium, corresponding to the short spondylium of Camarella vol- 

 horthi as illustrated by Hall and Clarke.*^ It differs from the latter in 

 the absence of a supporting median septum. 



Swantonia is closely related to CariiareUa., but it differs in the 

 absence of a supporting median septum beneath the spond3dium of the 

 ventral valve. No specimens of the dorsal valve have been observed. 



"Pal. New York, VIII, Pt. 2, pi. lxii, fig. 18. 



