Stratigraphic Occurrences of the Cretaceous Trigoniids in the Japanese Islands and their Faunal Significances

In this paper, the writer discussed briefly on the Trigoniid faunas of the Mesozoic era, especially Cretaceous period, in the Japanese Islands.
In Japan, Trigoniids appeared already in the Carnian of Southwest Japan and prospered in the Jurasssic to the Cretaceous. They are, however, completely lacking in the Cenozoic. Now, they are classified into 79 species and 10 subspecies in 21 genera in addition to 7 subgenera in 8 subfamilies.
The salient facts obtained through this study are as follows:
1. Although Trigoniids were rare in the Upper Triassic, some forms of Minetrigonia and Frenguelliella have hitherto been reported from several places in Southwest Japan. Triassic fauna may be related to one of the Arcto-Pacific region by the presence of Minetrigonia.
2. The Japanese Jurassic is characterized by abundant Vaugoiinae and Myophorellinae. The former flourished in the Lower to the Middle Jurassic and the latter in the Upper Jurassic. Jurassic fauna is probably connected to that of European rather than North American and Indian provinces because of the presence of common genera and subgenera as shown on Pl. 26, but Haidaia CRICKMAY, a subgenus of Myophorella, is characteristic in the Upper Jurassic of the Pacific regions.
3. Strongly carinate Trigoniids, i.e. Vaugonia s.l. and Myophorella s.l. etc., already disappeared in the Cretaceous when the Pterotrigoniinae and the Quadratotrigoniinae etc. took their place.
4. Cretaceous. Trigoniids most flourished in the Gyliakian when many genera, subgenera, and species occurred in various localities. In the Gyliakian, Heterotrigonia, Apiotrigonia, and Yeharella of Steinmannella inhabited in the Yezo geosynclinal region including Hokkaido and Sachalin, but they have hitherto been undiscovered from Southwest Japan.
5. In Japan proper, especially Southwest Japan, the Trigoniid assemblages of the Cretaceous were different distinctly in 3 series, i.e. Lower, "Middle", and Upper Cretaceous. The Neocomian is characterized by several forms. of Nipponitrigonia, Rutitrigonia, and Pterotrigonia s.s., while Apiotrigonia and Yeharella of Steinmannella flourished and restricted to occur in the Senonian s.l. Numerous Trigoniids are known in the "Middle Cretaceous" when 3 genera and 1 subgenus of the Pterotrigoniinae were existent. Scabrotrigonia and Acanthotrigonia are the most characteristics in this time.
6. In the Yezo geosynclinal region, Trigoniids are abundant in the Gyliakian formation of Ikushum-betsu in Central Hokkaido where Pterotrigonia s.s., Scabrotrigonia, Acanthotrigonia, Steinmannella (Yeharella), Apiotrigonia, and Heterotrigonia are co-occurrenced. In the Senonian s.l. the former three vanished but latter three survived.
7. The Lower and the "Middle" Cretaceous Trigoniids of Southwest Japan were allied probably to those of the Indo-African region rather than the North American because of the presence of Pterotrigonia s.1., Acanthotrigonia, and Rutitrigonia. While, the Upper Cretaceous ones were similar to those of the West Coast in North America.
8. KODAYASHi'S Trigoniinae are quite variable in surface costation on the flank and the area the latter of which is provided with radial or concentric costae near the umbo. With reference to the costation on the area Trigoniinae em. and Frenguelliellinae nov. can be distinguished from KOBAYASHI'S proposal. The latter typified by Frenguelliella LEANZA, ranges from the Carnian to the Senonian, but the former ranges the Anisian to the Senonian (?). The former flourished and inhabited in the Upper Jurassic to the Senonian of the Indo-African region, but the latter prospered and was prolific in the Middle Jurassic to the "Middle Cretaceous" of Eurasia.