394 Transactions. 



the Magellaninae, to species which come under Terebratella and Magellania 

 s. str., if these be defined by the type of hinge ; but there are species in each 

 of these genera which do not exhibit this property. Terebratella rubicunda 

 is nearly smooth, showing only a slight crinkling near the margin in old 

 shells, while Terebratella oamarutica Hutton is perfectly smooth. A Recent 

 species of Magellania from Macquarie Island collected on the Mawson Ex- 

 pedition by Mr. H. Hamilton is also perfectly smooth. Multicostation, 

 therefore, although constantly absent in Pachymagas and Neothyris, cannot 

 be considered a property on which to base generic distinctions. These 

 must rest primarily on the types of hinge parts and cardinal process.* 



Types of Hinge Parts and Cardinal Process. 

 («.) The Terebratelliform Type. Fig. 1. 



In M. fl,avescens, T. dorsala, T. sanguinea, and T. rubicunda — i.e., in 

 Magellania s. str. and Terebratella s. str. — the socket-ridges, hinge-plates, 

 septum, &c, are frequently described as lamellae, in reference to the 



fact that they are thin and 

 i ' ^>\ sometimes almost transparent. 



£ The hinge-plates are hollowed 



f^^? underneath (fig. 1, b), and if 

 they be broken away it is found 

 that the septum extends be- 

 Fig. ] .—Terebratella sanguinea (Leach), Wellington neath them right to the umbo 

 Harbour. Interior of dorsal valve, a. View of the valve. The cardinal 

 from above b. Front view of hinder end proce8S [ 8 superimposed on 

 broken oft, showing shell and septum in sec- f, , . , « f, , . 



tion/ lower sides of hinge-plates in perspective. the posterior end of the hinge- 



plates, and consists of a thin 



plate, broader than long, curving forward, and excavated on the upper 

 side to form a rounded cup or socket to which the diductor muscles are 

 attached. 



H HH (b.) Pachymagoid and Neothyroid Types. Fig. 2. 



HI In Terebratella parki Hutton and in other species referable to Pachymagas, 

 and in M. lenticularis and other species referable to Neothyris, the socket- 

 ridges, crural bases, and septum are thick and solid, and cannot be described 

 as lamellae. The socket-ridges nearly meet at the umbo, and thence diverge 

 more or less obliquely. The crural bases make junction with them on 

 their inner anterior corners, and are so intimately fused with them at this 

 point that they cannot generally be traced any farther posteriorly : but 

 in some species (N. lenticularis, N. oralis) they can be distinguished along 

 the inner sides of the socket-ridges for their whole length. The septum 

 does not extend as such to the umbo, but bifurcates posteriorly to join 

 the inner anterior ends of the socket-ridges (near the point of origin of the 

 crural bases as distinct processes). There are no definite hinge-plates as 

 in Terebratelliform types, their place being taken by the more or less steep 

 inner walls of the socket-ridges, which, with the bifurcating process of the 

 septum, enclose a trough which we may term the " hinge-trough.'" The 

 cardinal process rises from the bottom of the hinge-trough, and varies greatly 

 in different species. In New Zealand older Tertiary examples of Pachymagas 

 it is always fairly simple, but differs greatly in size and height. In its most 



* That is to say, these characters best serve to distinguish the evolutionary stocks. 



