HANNIBAL: CALIFOKNIAN FRESHWATER MOLLUSCA. 129 



early growth elongate-ovate, hiuge edentate and gently curved ; 

 habitat lacustro-fluviatile. 



Arnoldina, named after Dr. Ralph Arnold, includes, so far as 

 known, the sole species, Anodonta dejecta, Lewis, a mollusc which has 

 proved an anomaly to everyone who has studied it. The development, 

 peculiar beak-sculpture, and an indescribable velvety texture of the 

 shell preclude its reference to Anodonta or any allied genus. 



Arnoldina dejecta (Lewis). PI. VI, Fig. 9. 



Anodonta dejecta, Lewis, 1875, in Yarrow, 1875; A. Mearnsiana, 

 Simpson, 1893. 

 Shell as in genus ; inhabits ponds and sluggish streams. 

 Arizona System. 

 Quaternary : Le Conte Lake beds, California. 



Superfamily CYllENOIDE^ (Gray), 1840. 



The Cyrenoideoe, as here understood, embrace three families — the 

 Cyrenidce, Gray, 1840 (Corbiculidae, Gray, 1847), a comprehensive and 

 probably not entirely homogeneous group of brackish-water bivalves, 

 which does not concern this discussion particularly, the Sphooriidae, 

 Bourguignat, 1885 (Cycladae, Fleming, 1828), and the Corneocycladida3, 

 uov. (Pisidiadae, Gray, 1857), aquatic groups, both of which are 

 re])resented west of the Itocky Mountains. 



Tlie simplest forms in each family have, or at one time had, 

 rudimentary hinge-teeth, a quadrate-elliptical form, and low sub- 

 central umbones. From this type there has been a general tendencj^ 

 to develop a moderately complex hinge, the teeth corresponding to the 

 general formula — E. 101-0101-101, L. 010-1010-010 i — followed 

 ultimately by its degeneration, while a trigonal outline and high 

 sub-terminal umbones are acquired. In the Cyrenidos and Sphoeriida? 

 the umbones migrate toward the anterior extremity, a modification 

 analogous to that taking place in the Unionoideae and various marine 

 Pelecypoda, notably the Mytilidae and tlieir allies. In the Corneo- 

 cycladidae, however, the migration takes place toward the posterior 

 end. The cause is not understood, but there is reason to suspect it 

 to be produced in the same manner as the sinistral coiling of the 

 Gastropoda — the positions taken by the various cells in the early 

 cleavage stages of the developing embryo. 



An interesting feature of the study of the Sphteriidae and Corneo- 

 cycladido3, somewhat aside from the systematic phase, is the colour 

 of the epidermis. Anyone, with the assistance of a few drops of 

 oxalic acid, may convince himself that the shells of the members 

 of these families are uniformly a pale-straw colour. Deposited in the 

 epidermis, however, at the time of its formation, are generally varying 

 amounts of iron salts which produce the grey, green, brown, and 



^ Eupera, a tropical genus, has but a single somewhat bifid cardinal in each 

 valve in species seen by the writer, and the cardinals of the right valve of 

 Corneocyclas, sub-gen. Pisidiiim, are commonly united above. On the 

 other hand, the Amesodas show a tendency toward a bifurcation of the 

 cardinals. 



