BRACHIOPODA. 331 



the shell is traversed by small canals from one surface to the other, through which 

 little ccecal processes of the outer layer of the mantle pass, and are covered exter- 

 nally by a thickening of the epidermis. 



They have no special branchial apparatus. The respiratory function is per- 

 formed by the mantle, which is traversed by numerous blood-vessels. The arms are 

 frequently supported upon a calcareous framework on the interior of the dorsal valve, 

 as shown in the illustration of Waldheimia australis. The valves are opened by car- 

 dinal muscles, which originate on each side of the center of the ventral valve, and 

 converge toward the hinge margin of the dorsal valve, behind the dental sockets, 

 where there is usually a prominent cardinal process. The valves are closed by 

 adductor muscles, of which there are four in Crania and Discina. In many fossil 

 genera there are spiral processes, or loops, upon which are founded family distinctions. 



Shells are sometimes silicified, and become so transparent that they show the 

 coils when held up to the light. Sometimes the coils are preserved in empty shells; 

 and when shells are found wholly filled with spar, both valves may be removed, 

 and the sparry matrix scraped away on either side until the spirals may be clearly 

 seen by holding the specimen up to the light. 



The class was divided by King into two orders — the Clistenterata and Treten- 

 terata — which correspond with the Arthropomata and Lyropomata of other authors. 

 These divisions include the families as follows : 



Order Arthropomata. 



Athyridse, Atrypidse, Orthidse, Nucleospiridse, Pentameridse, Porambonitidse, 

 Productidse, Rhynchonellidse, Spiriferidae, Strophomenidse, Terebratulidse, Triplesiidse. 



Order Lyopomata. 



Craniidse, Discinidse, Lingulidse, Obolidse, Pholidopidse, J^Siphonotretidse, 

 Trirnerellidse. 



Family Athyridn. — Acambona, Athyris, Eumetria, Merista, Meristella, 



Whitfieldia. 

 Family Atrypidae. — Anazyga, Atrypa, Coelospira, Glassia, Koninckia, 



Zygospira. 

 Family Craniidn. — Crania, Pseudocrania. 



Family Discinid,e. — Discina, Orbiculoidea, Schizocrania, Schizobolus, Trematis. 

 Family Lingulidse. — Dignomia, Lingula, Lingulella, Lingulasma, Lingulepis. 

 Family Nucleosptridn. — Hindella, Meristina, Nucleospira, Retzia, {Trem- 



atospira. 

 Family Obolid^e. — Dicellomus, Elkania, Leptobolus,jLinnarsonia, Obolella, 



Obolus. 

 Family ORTHiDiE. — Meekella, Orthis, Orthisina, Skenidium, Vitulina. 

 Family Pentamerid^e. — Amphigenia, Anastrophia, Gypidula, Pentamerella, 



Pentamerus, Stenoschisma. 

 Family Pholidopid,e. — Pholidops. 

 Family Porambonitidse. — Porambonites. 

 Family Production. — Aulosteges, Chonetes, Productella, Productus, Stro- 



phalosia. 



