Ledge is not continued dorsally 7

7. With deep dorsal cavity; Genus _Citharistes_
secondary funnel not notched

No dorsal cavity; secondary Genus _Histioneis_
funnel deeply notched

* Presence at Woods Hole indicated by asterisk.


Genus EXUVIÆLLA Cienkowsky '82.

(Klebs '81; Pouchet '83, '86.)

The form varies from globular to ovoid, with occasionally a sharp
posterior end. Shells are usually somewhat compressed, and consist
of two valves, which frequently slide one over the other in such a
manner as to show the structure with great clearness. The right shell
may have a distinct indentation in the anterior edge. There are two
lateral, discoid, brown chromatophores, each of which possesses a
central amylum granule. The nucleus is posterior. Salt water.


Exuviælla lima Ehr. Fig. 18.

Synonyms _Pyxidicula_ Ehr.; _Cryptomonas_ Ehr.; _Prorocentrum lima_
Kent; _Amphidinium_ Pouchet.