CONTRIBUTIONS TO PALiEONTOLOGY. 



55 



cesses, extending into the cavity of the dorsal valve, gradually converge, 

 and are united by a transverse concave septum ; and beyond this, their 

 continuation forms the crura or spiral arms. 



The ventral valve has not shown dental lamellae, or processes of any 

 kind, beyond the converging edges of the shell beneath the beak. 



The following figures illitstrate in some degree the features above de- 

 scribed : 



Fig. 1. 



Fig. 2. 



Fig. 3. 



■'-^ 



Fig. 1. Retzia vera, Hall. Specimen natural size, showing the area and character of sur- 

 face. 



Fig. 2. An enlarged figure of the upper part of R. verneuili, showing form and proportions of 

 the area : /, foramen; a, area; w, the alation of the dorsal valve on each side of 

 the umbo. The same feature is shown in the dorsal valve of figure 1. The slight but 

 positive alation of the ventral valve, adjacent to the area, cannot be shown in the 

 figure without enlargement. 



Fig. 3. Transverse section of R. vera below the umbo, showing the diverging lamellae I and 

 transverse septum s uniting them, with their extension beyond (c) . 



In a single specimen longitudinally broken through, there is shown, on 

 one side, the broad subtriangular plate I, from the ventral margin of which 

 extends the concave or spoonshaped transverse septum s : this is united with 

 a similar plate on the other side, leaving between the outer shell and these 

 lamellae the space o, as shown in the transverse section. The continuation 

 of these plates in C apparently supports the spires, which are well shown 

 in another specimen. 



The specimens at my command are such as to afford but meagre materials 

 for illustration ; being all solid, and most of them distorted by pressure. 



Unfortunately I have no specimens of Retzia adrieni for comparison ; 

 but if we may infer its relations from the figures given, it is prolbably a 

 congener of the R. { T.) ferita which has the general features of Rhyn- 

 CHONELLA, and is certainly quite distinct generically from 7?, serpentina. 

 The shell of R. ferita is punctate, the beak of the ventral valve extended 

 and perforated at the extremity with a round foramen, below which there 

 is a narrow flattened area principally composed of two small deltidial 

 pieces.* 



* I have received from Dr. Rominger European specimens under the name of R. 

 ( T.) diodonta, which have the punctate structure and internal spires, and other cha- 

 racters in common with those designated above as R. ( T.) ferita, and which I have 

 supposed were authentic. The T. diodonta is regarded as a true Rhynchonella, and 

 by some European authors is cited as a variety of Rhynchonella borealis. 



