The genus Astarte, now limited to a dozen species in the North Atlantic and Arctic seas, has an almost world-wide geological distribution, and counts 200 species in d'Orbigny's catalogue, commencing with the has period. Crccssatella, now almost a southern form, is common in the cretaceous and tertiary strata of Europe. Closely allied to Astarte is the extinct genus Opis (fig. 16, 3), of which there are 42 species in the secondary series ; and Cardinia (fig. 16, 2), characteristic of the lias and oolites. The so-called ITnios of the coal measures (A nthracosia, King) are probably members of this One hundred species of Cardita (including Veneri eardia) are found in the secondary and tertiary strata ; of the 50 recent forms, one only is Arctic, and this occurs in the glacial deposits of England. The allied genus Myoconcha is characteristic of the older secondary rocks, and Hippo podium of the Has.
Secondary and Tertiary Bivalves.
4. Tancredia secariformis, Dkr.; Lisa, Saxony.
The Veneridce are pre-eminently characteristic of the tertiary and present period. Some obscure species of Venus are found in the oolites : Cytherea occur in the greensands ; Artemis, Trigona, Lueinopsis, Venerupis, and Tapes appear in the middle tertiary ; Petrieola in the eocene. The only extinct form is Grateloupia (fig. 16, s), which differs but little from Trigona.
The Mactras and Tellens are also comparatively modern groups ; most of the supposed oolitic species belong to Luci nidw, except Sowerbya (fig. 16, 5), which has a pallial sinus, and is found in the oolites of Mallon and Portland. Paso mobice and Mcsodesnice occur in the greensand ; Donax and Syndosmya in the eocene ; Gastrana, Venerupis, Lam.) and Lutraria in the miocene. Lutraria rugosa, still living on the coast of Portugal, is fossil in the raised beaches of Sussex.
The oldest forms of razor-fish (Solenidce) are those with the transverse internal rib (Soleeurtus), which occur in the neocomian, whilst true Solens and Glyeimeris appear first in the eocene strata. The genus Mya, as now restricted to the species resembling M. arenaria, are only met with in the newer tertiary. Corbula ranges upwards from the lower oolites ; Neara appears in the upper greensand ; and Thetis Poromya, Forbes) in the neocomian.
Above 100 species of Panopcea (a genus essentially like Mya) have been obtained from oolitic and tertiary strata in all parts of the world. They are with difficulty distinguished from those equally numerous forms of Anatinidce which have been associated with Pholadomya on account of the tenuity of their finely-granulated valves ; they constitute the genus Mya,eites (Bronn), and occur in all the paheozoic and secondary rocks ; some of the oolitic and cretaceous species are distin guished by V-shaped furrows (fig. 16, 6). Still more numerous are the fossil forms of Pholadomya, which range upwards from the liar, but are reduced to a single species now living in the Caribbean seas. Shells with the umbones fissured like Anatina also occur in the oolites. Pandora first appears in the older tertiary. Amongst the extinct genera referred to this family are the Silurian Grammysia (fig. 13, so), with valves folded transversely ; the carboniferous Edmondia (fig. 13, n), with large oblique cartilage plates ; and Cardiomorpha, shaped like Isocardia ; and the oolitic Cercrmya (Ag.), which also resembles the heart-cockle in form. Cercamya is an oolitic Anatina, with the posterior end of the valves much attenuated.
The genus Gastrocheena appears in the lower oolites ; and casts of its burrows are frequently preserved after the decom position of the coral in which they were made. Clavagella dates from the upper greensand, and Aspergillum from the miocene. Saxicava is found in the newer tertiary and raised beaches of Northern Europe ; and the great species commonly called " Panopoea" Korwegica is a characteristic fossil of the newer pliocene of Britain and Greenland.
The Pholades and ship-worms appear first in the oolitic strata. Forms resembling the recent Martesia striates have been discovered in fossil wood of the liar and Speeton clay. Jouannetia (Desm.) was first known as a miocene fossil ; and Pholas occurs in the older tertiary. Extinct species of Teredo are found in the silicified wood of the greensand of Blackdown and in the fossil palm-fruits of Brabant and Sheppy. The drift-wood of the London clay is usually perforated by the ship-worm, and also by an extinct form (Teredina, fig. 16, 9), which resembles Martesia in possessing an umbonal shield : when adult it not only closes the anterior pedal opening, but also cements its valves to the shelly lining of its burrow, like an Aspergillum. Specimens have been obtained in which the whole interior of the valves and tube had been excessively thickened towards the close of life by successive layers of shell.