THE BRITISH EOSSIL BRACHIOPODA. 321 



shell are lined by the internal layer of the mantle, with the exception of the muscular 

 impressions, or those portions where the muscles are inserted on the interior surface of 

 the shell." 1 The outer layer closely lines the inner surface of the valves, to which it 

 sometimes adheres ; and in those species in which the shell is traversed by canals, there 

 exists on the surface of the mantle facing the inner surface of the valves corresponding 

 short, cylindrical, membranaceous projections or caeca, inserted into the tubular orifices 

 traversing the shell. These caecal prolongations do not exist in those genera (such as 

 Bhynchonelld) where the shell has no tubular perforations. The inner layer is rather 

 thicker than the outer one, and is covered with vibratile cilia. Between the two layers 

 composing the mantle are, as we have already said, the blood-channels or lacunas. These 

 vary in their disposition in different genera ; as they project to some small extent, they 

 leave corresponding indentations or depressions on the inner surface of the shell, so that 

 their shape and directions can very often be traced on the fossils of extinct genera and 

 species, as well as if the animal were still living. A knowledge of the characters and 

 variations assumed by the vascular system in the Brachiopoda is therefore of great 

 importance, especially in the study of extinct forms. 



In some groups there are four principal arterial trunks in the dorsal lobe of the 

 mantle ; and these in most cases run direct to the front (at any rate, the central pair), 

 and bifurcate at intervals. The other two or outer vessels, in some genera, deviate 

 at about half their length and curve round towards the cardinal edges, encircling 

 the ovarian spaces and giving off numerous branches, which bifurcate several times 

 from their outer margin and extend all round the margin of the mantle. According 

 to Cuvier, the first indication of a special breathing organ is presented in Lingula, in 

 which the veins develope parallel rows of small vascular processes. The veins, as 

 stated by S. P. Woodward and others, open into the visceral cavity, which is in itself a 

 great vascular sinus. 



Examples of these different arrangements of the vascular system will be seen in the 

 pages and plates of this Monograph, as well as in the works of other palaeontologists. 

 I have lost no opportunity of representing them where they came under my notice. 

 They are very often observable on the well preserved surface of internal casts, which in 

 reality represent the surface of the mantle itself. For this reason well preserved internal 

 casts should always be sought for and carefully examined. 



Hancock observes, and the same has been noticed by several other zoologists, that the 

 inner lamina of the mantle, and more particularly that portion of it forming the floor of 

 the great pallial sinuses, undoubtedly assists in the purifying of the blood. The mantle- 

 lobes are therefore not only organs by which the shell is formed and repaired, but are 

 subservient to the process of respiration and circulation of the blood, and sometimes 

 to the ejection of the eggs which accumulate in the larger sinuses of the arterial system. 



The Brachiopoda are extremely prolific, and their innumerable ova are spherical or 

 1 ' Reclierciies sur l'organisation du Manteau chez les Bracliiopodes articules,' Caen, 1864. 



