v. 
Silurian, The Brachiopoda are amongst the most numerous and 
characteristic of the Paleozoic series. The Pelecypoda first appear 
in the Tremadoc rocks, sparingly in the Silurian, becoming very 
numerous in the Mesozoic, and have held their ground to the 
present day. The Gastropods have also a very wide distribution 
in time, from the Arenig, or Skiddaw series, to the present day. 
The characteristic families of the Pelecypoda cf the Lower 
Silurian are Aviculide, Mytilide, Arcide, Nuculide, Astartide, 
Lucinide, Cardiide, Pholadomyide. The Gastropoda are Euom- 
phalus, Murchisonia, Natica, Pleurotomaria, Trochus. Old types 
gave way to new inthe Triassic age, when the Ostrea appeared for 
the first time, with Avicula, Lima, Pecten, Arca, Astarte, Cardium, 
Modiola, Trigonia, Pholadomya, among the Pelecypoda ; Alaria, 
Cerithium, Chemnitzia, Natica, Pleurotomaria, Trochus, Turbo, 
among the Gastropoda. Their development culminated towards 
the end of the Jurassic age, when, climatal changes taking place, 
lowered the temperature, and to which may be assigned the dis- 
appearance of the great Saurians which had previously swarmed 
on sea and land. 
The Tertiary age shows a gradual change, and an approach 
towards the Molluscan life of the present day. Some genera 
disappeared altogether. Until within the last 50 years Phola- 
domya was supposed to be extinct, it is now found living in the 
Australian Seas. Pecten, Mytilus, Avicula, and Nucula have 
continued almost without any modification from Paleozoic ages. On 
the other hand Caprinide and Hippuritide were restricted to the 
Cretaceous age. The Fresh-water Gastropoda, Neritina, Planorbis, 
Paludina, Hydrobia, Melania appeared for the first time in the 
Oolitic age ; Limneus, Physa, Bithinia, in the Purbecks and 
Wealden. Gastropoda are rare in the Lower Chalk and not 
well preserved. The fresh-water and land-shells began to take a 
prominent place towards the end of the Cretaceous age, and have 
continued to the present day. 
Most of the genera found in the Eocene and the Oligocene beds 
are now living, but the species differ. The marine Miocene 
