76 



. 



depressions till the adjoining plaits meet and open into each 

 other : processes conspicuous : wall thick. 



The mode of fold in this species resembles that of Ventriculites 

 bicomplicatus in the fact of being repeated on the plaits of each 

 surface. It differs essentially, however, in the fact that the de- 

 pressions, though generally round, are, on neither surface, at re- 

 gular intervals : consequently no regular figure is assumed in the 

 general aspect of either surface. 



I have named the species alternans from the circumstance of 

 the repetition on the two surfaces of the same manner of fold ; 

 while the straight plait is clearly traceable in the central portion 

 of the wall. A transverse section of a specimen of this species is 

 seen on PI. VII. fig. 2. Its difference from a similar section of 

 C. longitudinalis is very marked. 



This is an extremely rare species. 



5. Cephalites bulletins. PI. VII. fig. 3, & PI. XIV. figs. 6 & 7. 



Plaits broad and deep : outer plaits raised in large and very pro- 

 minent projections at considerable intervals, and in such man- 

 ner that they range spirally round the whole body : projections 

 nearly lozenge-shaped and terminating abruptly in an almost 

 flat and somewhat expanded top, having a slight depression from 

 the upper angle towards the middle : inner plaits having large 

 circular depressions at equal intervals ; bulging on each side 

 around depressions till adjoining plaits meet and open into 

 each other : processes very conspicuous : wall very thick. 



This is a most curious and interesting as well as rare but well- 

 marked species. The depressions on the inner folds are much 

 larger than in C. guttatus, which latter have been seen to be 

 larger than in C. longitudinalis. But the external fold is the 

 most deserving of attention. When the specimen is first opened 

 there are seen only a number of nearly semilunar marks. On 

 carefully applying the point of the knife it is found that this se- 

 milunar appearance is caused by very prominent projections, the 

 tops of which are all closed, but have a partial depression at their 

 upper extremity, and which depression is filled as usual with the 

 matrix. The projections themselves are of large size, measuring 

 about two lines in their longest diameter. They stand out nearly 

 or quite half the thickness of the wall, which is generally four 

 lines thick (see fig. 3. PI. VII.*). They differ widely from any- 



* This is a longitudinal section taken rather obliquely in order to preserve 

 the roots. It is not quite regular therefore ; but, on the side which is pre- 

 served, the projections can be well distinguished. 



