Mesodfsma.} PELECYPODA. 961 



Hob. North and South Islands ; Chatham Islands ; on sandy 

 flats between tide-marks ; common. Brought to England by Captain 

 Cook. 



Maori. Pipi, kokota, (teste Hutton). 



Fossil. Miocene and Pliocene. 



Var. aucklandicum, von Martens, 1879. 



Mesodesma aucklandicum, von Martens, S.B. Ges. naturf. Freunde, Berlin, 

 1879, 37. 



Distinguished from' the species : The shell is much larger and 

 ponderous, more ventricose, of a dark-fuscous colour, the posterior 

 end hardly higher than the anterior ; the hinge is still more concen- 

 trated, the lateral teeth stouter and slightly shorter. 



Length, 67 mm. ; height, 39 mm. ; diameter, 23 mm. (type). 

 Length, 90 mm. ; height, 54 mm. : diameter. 32 mm. (specimen 

 collected by Captain Bollons). 



Type in the Kgl. Museum fur Naturkun.de, Berlin. 



Hob. Auckland Islands (Hermann Krone, Captain Bollons). 



Fam. MACTRIDj, Gray. 



Animal with the external branchial plate directed ventrally ; 

 siphons united, more or less invested by a chitinous sheath ; foot 

 long, stout, bent at an angle, and without a byssus. 



Shell porcellanous, with an obvious epidermis, usually rounded 

 triangular, with smooth or concentrically sculptured surface, smooth 

 margins, and prominent prosogyrous beaks ; valves equal, free, usually 

 with a slight posterior gape ; area not limited ; ligament variably ex- 

 ternal or internal ; resiliurn connecting subtriangular chondrophores 

 usually excavated out of the hinge-plate, rarely with a prop or buttress ; 

 hinge-plate well developed, with normally an anterior and posterior 

 lateral in the left, received into sockets or paired laminae in the right 

 valve, or obsolete ; cardinals in the right valve 2, with their dorsal 

 edges usually soldered together, and 1 bifid or deltoid cardinal in the 

 left, fitting below the former, a delicate accessory lamella often present 

 in either valve, or all may be more or less obsolete. 



Cretaceous to Recent. 



Subfam. MACTRIN/E. 



Siphons partially or wholly naked, wholly retractile within the 

 shell ; mantle, between siphons and anterior adductor, chiefly open 

 ventrally. 



Shell sub equilateral, nearly closed. Hinge : In the left valve an 

 anterior and posterior lateral lamina, and a bifid or A-shaped cardinal 

 tooth in front of a pit for the resiliuni ; above the latter a scar or 

 surface of insertion for the ligament. In the right valve 2 anterior 

 and 2 posterior laminae, between which the laterals of the opposite 

 valve are received ; 2 lamellar cardinal teeth, inclined to each other 



31 Moll. N.Z. 



