210 



ritOCEKDINGS OF THE MALACOLOGICAL SOCIETY. 



Subfamily EUVILIINiE. 



Shells small, thin, equilateral ; ligament marginal, obsolete or 

 absent ; resilium small ; hinge much concentrated ; the lateral teeth 

 small ; the dorsal anterior lamina absent, the ventral more or less 

 coalescent with the anterior arm of the right cardinal; left cardinal 

 large, bifid ; pallial sinus well marked. 



There is some doubt as to the characters of the soft parts of Ccecella, 

 but so far as the shell goes it is simply a lai'ge brackish-water Ervilia, 

 and if the latter belongs to the Mesodesmatida) so docs the former. 



There are some curious parallelisms in the characters of the diiferent 

 groups above mentioned. They cannot all be tabulated in one scheme, 

 but offer some points worthy of investigation. 



Theoretically, after the division of the primitive ligament into 

 ligament and resilium, the order of modification sliould have followed 

 two lines — one in which the parts, though separated, are not walled 

 apart by a shelly septum, and hence the possibilities of modification 

 are still left unlimited, and the other, where the separation has been 

 made final by the development of a shelly partition. In accordance 

 with this hypothesis we find the oldest fossil Mactridaj Spisuloid and 

 even without a solid base of attachment for the ligament proper ; later 

 this base is supplied, and certain forms assume the Mactroid type. 

 Fteropsis is the earliest known genus of its subfamily, and is Spisuloid, 

 while the recent Rmta is Mactroid. The older Lutraria, and the 

 newer Tresus, the ancient Standella, and the modern Heterocardia, 

 offer parallel cases. The Zenatiinte appear to be a relatively modern 

 type, and are all Spisuloid ; as is Anatinella. In each case the 

 Mactroid features belong to the later stage of each group, when they 

 are present at all, some groups not having yet acquired the requisite 

 equilibrium. The Mcsozoic forms, so far as known, have the Vymbo- 

 phora hinge ; the Eocene ones, in America, are Spisuloid. Later, 

 in the Miocene, comes total submergence in Mulinia and Rangia, 

 while none of the aberrant later developments have reached the 

 Mactroid stage. These features may be tabulated as follows, for 

 the Mactrida) : — 



Ligament : Spisuloid. Submerged. Mactroid. 



Spisu/a. Mulinia. Muctra. 



Spifudu. Mangia. Mactra. 



■XT L ■ \ Hemimactra. Jiatu/iaiiel/a. Mactrotoma. 



JMactnua; ^ Cyinhophura. llivranyia. Muctmkrma. 



Scliizudesma. — C'celoinactra. 



Leptuspinula. — Mactrclla. 



1 Pteropsis. — Lahiosa. 



Pteropsidinac |- — — liaeta. 



J — — lia'ctcUa. 



I Lntraria. — Tresus. 



Lutrariinic - IStandalla. — Jleterociirdia. 



\ Eastonia. — — 



IZenatia. — — 



liesania. — — 



Darina. — — 



Anatinelliuac Ana/inella. — — 



