Chapter E adds 112 described species and subspecies (a few briefly described) to the some 440 covered in preceding chapters : 27 additional gastropods, 18 scaphopods, and 67 pelecypods in 10 families. It is estimated that about 125 species are to be added in chapter F, the final chapter.

The Arcidae is by far the largest family in the pelecypods so far studied : 30 species in six genera. The genus Anadara is the largest in that family : 25 species, representing the subgenera Hawaiarca?, Rasia, Tosarca, Grandiarca, Potiarca, and Cunearca. Potiarca, based on a living western Pacific species, is adopted for many American species heretofore assigned to Cunearca.

Eighteen of the 27 additional gastropods were found at a new locality in the late Eocene part of the Gatuncillo formation. They 'are assigned to 16 genera, 14 of which are not represented at other Gatuncillo localities. The genera include Faunus, Turnpanotonos, and Bezaneonia, which are rare in America, and the rare endemic American genus Harrisianella.

The marine member of the Bohio (?) formation is now assigned to the late Eocene, instead of late Eocene or early Oligocene—the only change in age assignment. Eocene affinities are especially strengthened by the presence of Samanoetia samanensis and Volvariella?, if indeed the unnamed species represents that genus, as seems likely. Both the marine member of the Bohio( ?) and the late Oligocene part of the Bohio contain early small species of Anadara, subgenus Rasia.

The fossils from the moderately deep-water facies of the late Oligocene Caimito formation include the earliest hexagonal Dentalium of the subgenus Dentalium s.s., from the American mainland, and a widespread and abundant species of the essentially deep-water subgenus Fissidentalium.

Anadara chiriquiensis chiriquiensis, a relatively small form of the subgenus Grandiarca that is widely distributed in the Miocene Caribbean province, occurs in the Culebra formation. For the most part it indicates brackish water.

Among the fossils from the La Boca formation proper, Cyphoma aff. C. intermedia is the earliest representative of an endemic American genus, and Isognomon mimeticus, a remarkable, narrowly mytiliform species is the type of the new subgenus Mimonion. Both the Culebra and La Boca, both of early Miocene age, contain predecessors of Gatun species.

As in other chapters, the largest number of species is from the middle Miocene Gatun formation: 50 species. Bailya crossata is the first Tertiary species of the genus from the Caribbean region, and the earliest now known. A bizarre, incomplete gastropod is described as xancid?, genus?. The Gatun contains 13 of the 24 species of Anadara and half of the 12 species of the subgenus Rasia of that genus. In consideration of two battered and worn specimens of a large form of the subgenus Grandiarca, identified as Anadara grandis patricia?, the similar Tertiary forms in the Caribbean region are reviewed and divided into a brackish-water group and a marine group. The Gatun fauna now totals some 330 species.