BRACHIOPODA. 213 



Recent Discince often have minute fry attached to their valves, and Mr. 

 Suess, of Vienna, has noticed a specimen of the fossil String ocephalus, which 

 contained numerous embryo shells. 



Nothing is yet known respecting the development of the Brachiopoda., 

 but there can be no doubt that in their first stage they are free and able to 

 swim about, until they meet with a suitable position. It is probable that in 

 the second stage they all adhere by a byssus, which in most instances becomes 

 consolidated, and forms a permanent organ of attachment. Some of the 

 extinct genera (e. g. Spirifera and Strophomena) appear to have become free 

 when adult, or to have fixed themselves by some other means. Four genera, 

 belonging to very distinct families, cement themselves to foreign objects by 

 the substance of the ventral valve. 



The Lamp -shells are all natives of the sea. They are found hanging 

 from the branches of corals, the under sides of shelving rocks, and 

 the cavities of other shells. Specimens obtained from rocky situations 

 are frequently distorted, and those from stony and gravelly beds, where there 

 is motion in the waters, have the beak worn, the foramen large, and the 

 ornamental sculpturing of the valves less sharply finished. On clay beds, as 

 in the deep clay strata, they are seldom found ; but where the bottom con- 

 sists of calcarious mud they appear to be very abundant, mooring themselves 

 to every hard substance on the sea-bed, and clustering one upon the other. 



Some of the Brachiopoda appear to attain their full growth in a single 

 season, and all, probably, live many years after becoming adult. The growth 

 of the valves takes place chiefly at the margin; adult shells are more 

 globular than the young, and aged specimens still more so. The shell is 

 also thickened by the deposit of internal layers, which sometimes entirely 

 fill the beak, and every portion of the cavity of the interior which is not oc- 

 cupied by the animal, suggesting the notion that the creature must have died 

 from the plethoric exercise of the calcifying function, converting its shell 

 into a mausoleum, like many of the ascidian zoophytes. 



The intimate structure of the shell of the 

 Brachiopoda has been investigated by Mr. Morris, 

 Prof. King, and more recently by Dr. Carpenter ; 

 according to the last observer, it consists of flat- 

 tened prisms of considerable length, arranged parallel 

 to each other with great regularity, and obliquely 

 to the surfaces of the shell, the interior of which is 

 imbricated by their out-crop (fig. 110.) This struc- 

 tructure only is found in the Rhynchonellidee; but 



in most perhaps all the other Brachiopoda* pj gt HQ Ttrebratula. 

 the shell is traversed by canals, from one surface 



* The fossil shells of the older rocks are so generally pseudomorphous, or par- 

 take of the raetamorphic character of the rock ttself, that it is difficult to obtain sped 

 mens in a state fit for microscopic examination. 



