38 THE BEACHlOPODA. 



37. RHYNCIIONELLA SUBRINGENS, Davidson. 



1851. KHYNCH. SUBRINGENS, Dav., British Brach. Palseont. Soc., 

 plate xiv., fig. 17. 



The types of this peculiar little species are totally distinct from 

 typical Rhynch. suldecorata young forms (Dav. British Brach. 

 Appendix, plate A, figures 24 and 26), but varieties of both 

 occur which seem extremely hard to separate one from the other. 

 Rhynch. subringens is globular, on the lesser valve it has one 

 strong mesial rib, somewhat raised, and two distinct ribs on each 

 side also rather strong and well marked. On the larger valve it 

 has a deep sinus in the middle, and three ribs two distinct and 

 one rather small on each side of this. The edges of the valves 

 come together at an acute angle. Rhynch. suldecorata, however, 

 is not nearly so globular, has the mesial fold a good deal raised 

 and divided by a sinus into two parts, and it has three ribs on 

 each side, which are not nearly so prominent as the two in R. 

 subringens. On the larger valve it has a mesial sinus, down the 

 middle of which runs a small rib. On each side of the sinus are 

 four rather small ribs. None of the furrows in Rhynch. suldecorata 

 are nearly so deep as those in Rhynch. subringens. The variety of 

 Rhynch. subringens is very globular, larger than the type, has the 

 mesial fold undivided, and possesses two more small ribs on each 

 valve. Edges of valves joined more or less obtusely. The variety 

 of Rhynch. suldecorata is also nearly as globular as Rhynch. sul- 

 ringens, as the mesial fold is divided by a small sinus, and a small 

 corresponding rib down themiddle of the largervalve. Itwillthus 

 be seen that the varieties of Rhynch, subringens and Rhynch. sul- 

 decorata approach each other very closely, in general form 

 especially, and are only separable on account of the division of 

 the mesial fold. The similarity, however, of the variety of Rhynch. 

 subringens with the enlarged figure given by Mr. Davidson, plate 

 xiv., fig. 17, is remarkable. 



Dimensions. Type form Length, 3 J ; breadth, 3 ; depths 

 lines. Another specimen Length, 5 ; breadth, 5 ; depth, 5 

 lines. 



