120 Mr. H. Seeley on Cambridge Palaontology ;— 



bones level with tlie anterior side, and (apparently) recurved. 

 Anterior side abrupt and rounding ; the posterior sloping and 

 compressed. From the top of the umbo descends a wide 

 sulcation, which gradually widens until it equals, at the base, 

 a fourth of the shell's length : it is nearer to the anterior than 

 to the posterior end. The elevation which bounds it on the 

 posterior side is more prominent than that on the anterior. 

 The sulcus and adjacent part of the shell are greatly thickened 

 Lnternally, so that in the cast the hollow is very deep. From 

 the under side of the umbo descends a ridge, which terminates 

 at the point where the straight posterior end makes a right 

 angle with the base. 



The characters which have induced me to place this remark- 

 able form provisionally with the Arks are its shape, posterior 

 ridge, (apparently) straight hinge-line, and unsinuated pallial 

 impression. The position of the muscular scars, the posterior 

 of which is near the hinge-line, the angular bend in the pallial 

 impression, and the dorsal thickening, might, with some of the 

 foregoing, be thought sufficient characters to place it in or near 

 to the AnatinidaB. Such a combination of characters would 

 alone render its position perplexing ; but between the anterior 

 muscular impression and the bend which the anterior side 

 makes with the hinge-line are two folds, reminding one of the 

 Rudistes. Additional specimens may give its generic characters 

 and affinities. 



Unique. Collected by Mr. W. Farren. 



Nucula subelliptica . PI. VI. fig. 4. 



Form subelliptical, slightly elongated, rather compressed. Urn- 

 bones central ; hinge-line on each side straight, equal, and 

 similar. Lips smooth. Apical angle about 135°; width 

 7 lines ; length 6 lines. 



This species is intermediate in form between the N. Vibrayeana 

 (D'Orb.) and N. subrecurva (Phil.) ; from the former it is distin- 

 guished by the elongated form, and from the latter by its width 

 and similar sides. 



Not common. Shell unknown. Coll. University ; J. Carter, 

 Esq. ; Mr. W. Farren. 



Nucula rhomboidea. PI. VI. fig. 5. 



Form a lozenge, slightly elongated, nearly symmetrical, com- 

 pressed. The four sides nearly equal ; angles, except the 

 apical, rounded. There are indications of a lunule. Owing to 

 the angle opposite the apical being rounded, as well as those 

 at the ends, the halves of the base are not so straight as are 



