of the Ventriculidse of the Chalk. 



367 



This form will readily be distinguished from every other not only 

 ;y the difference in the primary fold from that of any species which 

 I pproach it in the character of the brachial fold, but in the essential 

 (liaracter of that brachial fold itself. The narrowness, in com- 

 l)arison with the length, and the close setting of the tubes, are 

 together found in no other „. ^ 



species, ihe accompany- *' 



iiig figure, which is a trans- 

 \ erse section of a specimen 

 of this species, near the top, 

 und just therefore missing 

 tlie central cavity, could 

 be presented by a similar 

 section of no other species. 

 It seems to me that there 

 is generally no marked di- 

 stinct opening to the cen- 

 tral cavity, but that it is 

 surrounded on all sides by 

 the tubes. In fig. 7 of 

 PI. XV. the upper part was 

 cut away before it came into my possession, but the wall of the 

 cavity is, in that specimen, rounding inwards, and not expanding. 



It is important to notice that each brachial fold is distinctly 

 tubular and prominently projecting, and that its termination is 

 slightly contracted, in the present species, two characters in which 

 it essentially differs from B. labrosus and B. fenestratus, and cha- 

 racters which must obviously have materially affected the access 

 and circulation of the sea-water. The central cavity into which 

 those tubes open is another character to which the last remark 

 strongly applies, and is a character also at once distinguishing 

 this species from B. fenestratus. 



This species is from the Middle Chalk. 



5. Brachioliies fenestratus. PI. XVI. fig. 3. 



Membrane simple and without any primary fold : brachial fold 

 in narrow tubes anastomosing and opening into each other in 

 not very regular figure, but in several vertical and horizontal 

 planes, leaving interspaces between them about equal to their 

 own width : each tube rounding at mouth and projecting 

 slightly beyond the plane of the most extei'nal range of the 

 anastomosed mass. 



, The description will at once show wherein this species differs 

 prom the last. A single fragment of tube may be mistaken, as 

 the primary fold is not strikingly marked in either, and the size 



