78 PALEONTOLOGY. 



Myacites (Bronn), and occur in the palaeozoic and secondary 

 rocks; some of the oolitic and cretaceons species are distin- 

 guished by V-shaped furrows (fig. 20, 6). Still more numerous 

 are the fossil forms of Pholadomya, which range upwards from 

 the lias, 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. 17, !°), with valves folded 

 transversely; the carboniferous Edmondia (fig. 17, "), with 

 large oblique cartilage plates; the palaeozoic Cardiomorjiha, 

 shaped like Isocardia; and the oolitic Keromya (Ag.), which 

 also resembles the heart-cockle in form. Cercomya is an oolitic 

 Anatina, with the posterior end of the valves much attenuated. 



The genus Gastrochcena 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. Glavagella 

 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 " Panopwa" Norwegica 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 striata have 

 been discovered in fossil wood of the lias and Speeton clay. 

 Jouannetia (Desm.) was first known as a miocene fossil; and 

 Photos 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. 20, 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 



