18C "TERRA NOVA" EXPEDITION. 



All three examples show incipient ventral uniplication. 



The valves have a smooth surface, hut at the anterior margins of specimens No. 1 

 and No. 2 incipient alternate multicostatiou is visible as a slight crinkling of the edges 

 of the valves. On specimen No. 2 the dorsal valve clearly shows two costoe occupying 

 the sinus. 



The shell-structure is conspicuously and evenly punctated. On the inner surface 

 the pores are circular ; on the outer, they are slightly oval and larger than the inner. 

 The number of pores per square millimetre, at the middle of the ventral valve (specimen 

 No. 1), ranges from 180 to 200 (average often counts = 188). 



In the interior of the dorsal valve (No. 1, 3' 5 mm. long) the loop consists of two 

 very thin descending branches and an ascending portion in the form of a ring, which is 

 broad below and narrower above [PL I, fig. 7]. Both the descending and ascending 

 portions are united along the side of the septum, and the stage of loop-development 

 is not unlike that designated by Thomson [1915 (2), p. 405, fig. 6] " Magelliform " 

 for Tert'liratelli rubicunda. The anterior part of the high septum, however, is 

 produced somewhat beyond the broad base of the ring, as in the Magadiform stages 

 of TercbmtL'lla dorsata and Neothyris lentieuhtris, figured by Beecher* [1893, pi. i, 

 figs. Ea and Eli]. 



The septum, which is extremely thin anteriorly, broadening rapidly posteriorly, 

 reaches right back to the hinge-plates, which consist of two oblique lamellae extending 

 from the dental socket-ridges towards the centre-line of the valve. These plates arc 

 hollowed out underneath in the direction of the apex. Between the hinge-plates is a 

 somewhat narrow depression or trough which extends forward along the upper surface 

 of the septum as a shallow groove. In the centre of this trough, between the hinge- 

 plates, is a small elongated tubercle. The cardinal process consists of a transverse 

 bilobed plate superimposed on the inner posterior ends of the hinge-plates. The socket- 

 ridges overlap the margin of the valve posteriorly as two tiny ears. 



The presence of a groove along the top of the septum and of a tubercle in the 

 hinge-trough are interesting points. I have met with the same features in juvenile 

 stages of other species of the 3[aiidlania-Tnre]>mteUn group (<'.</., T. dorsata). The two 

 edges bounding the groove are distinctly connected and continuous with the hinge- 

 plates ; the tubercle is apparently a disconnected part of the septum, or the beginning 

 of a buttress to the cardinal process. In an early stage of Magellania flaveseens the 

 tubercle looks as if it were the posterior part of the mesial septum protruding through 

 the line of joining of the hinge-plates. In later stages of the same species it usually 

 disappears, but occasionally in fully adult specimens a distinctly bulbous cardinal 

 process with a triangular buttress is seen extending forward into the hinge-trough. 

 Similar features are also present in T. <lor*atn and M. ;v//<w/. One or two of my 



* In this stage, apparently, the descending and ascending branches have not yet united on the side 

 of the septum. 



