LECTURES ON MOLLUSCA. 273 



very large a proportion of the fossil bivalves, up to the tertiary age, 

 belong to this class. 



The structure of the shells is more simple than in the ordinary 

 bivalve and univalve tribes. There is no distinction between the outer 

 and inner layers ; the whole consisting of long flattened prisms, arranged 

 sideways. In most of the families these are traversed by numerous 

 vertical tubes, which are trumpet-shaped outside and sometimes arbo- 

 rescent. As the valves open but a little way, and there are no specially 

 directed breathing currents, the tubes which are no doubt occupied by 

 prolongations from the mantle (which is not loose, as in ordinary 

 bivalves) assist greatly either in the breathing or excretory functions. 

 There are no pores in the internal lip skeleton. 



In the ancient rocks both of the Old and New World, a Lingula is 

 the first organic "footprint on the sands of time/' the same generic 

 form being still found in all the oceans of the globe. As we read on- 

 wards in the palasozoic chronicles, the forms, and still more the number 

 of specimens, continue prominent, typical, and diagnostic above all 

 other fossils until they reach their maximum of development in the 

 Devonian ages. They continue extremely abundant throughout all 

 the secondary and cretaceous ages ; decreasing in comparative import- 

 ance as the Lamellibranchs gradually appear. The Productus tribe 

 does not enter the secondary period ; the Spirifers and Orthids die out 

 in the lower beds; while the JihynconeUids, Craniads, and Lingular 

 have maintained their position, throughout all the changes in other 

 races of animals, throughout all the fossil ages, to the present time. 

 The Terebratidids were the latest to appear, not showing themselves 

 decisively till the carboniferous age. Most of the tertiary and living 

 forms belong to this group. Although the Palliobranchs are compara- 

 tively rare in the tertiary ages, the boreal Crag furnishes us with one 

 of the largest species known. No members of this class attain the 

 size of the Lamellibranchs ; a more complete system for breathing and 

 digestion being necessary to maintain a Scallop, a Panopasa, or a 

 giant clam. 



It used to be thought that the prevalence of Palliobranchs in any 

 stratum was a sure evidence of deep-sea origin. It is true that they 

 are found living in the greatest depths yet dredged ; but species are 

 also found in pools left by the retiring tide ; and it is probable that 

 many of the earliest rocks were deposited in comparatively shallow 

 water. Although the recent shells are still rare in collections, they 

 are common in the regions they inhabit ; and as seventy species are 

 already known, a greater number than has been discovered in any 

 single secondary stratum, and as probably "more than half the living 

 forms are yet to be discovered, we have no right to say that the race 

 are dying out. While some species are very local, other forms are 

 widely diffused both in area and in time. The Atrypa reticularis is 

 found through a whole series of strata, in the Old and in the New 

 World t ; and Spirifcra striata ranges from the Cordillera to the Ural 

 mountains. 



The. fullest account of the shells and physiology of this class will be 

 found in Davidson's treatise on the "British Fossil Brachiopoda/' 

 printed by the Paleeontographical Society. A very full abstract of 

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