CLASS IE 



BRACHIOPODA 



357 



(Chonetes, Produdus) or be cemented by the whole or a part of the surface of 

 the ventral valve {Crania, Davidsonia, Thecidea, Streptorhynclms). In some cases 

 (Glottidia and Lingula) Brachiopods live throughout life partially buried in the 

 sand or mud of the sea-bottom. 



Ornamentation. — The external form and ornamentation of the shell afford 

 important characters for determining the species. The anterior margin of 

 one valve is frequently indented by a median sinus, and the other usually 

 exhibits a corresponding /oZ(/, or elevation. 



In the earliest shell growth stages Brachiopod shells are invariably smooth, 

 and may remain so throughout life, but the greater number develop radial 

 striae, ribs or undulations, and these are usually crossed by concentric growth 

 lines, or lamellae, which are sometimes of great width, or may be extended into 

 spines. There may be more or less long tubular spines scattered over one or 

 both valves, or sometimes restricted to a single row along the cardinal line. 

 Under the term Loricatae, Leopold von Buch included all Brachiopods in which 

 the radial folds, or costae, are arranged in regular succession in such manner 

 that elevated ridges at the anterior margin of one valve coincide with the 

 indentations of the other. In the Biplicatae, a median fold or sinus is 

 bordered on either side by a broad fold. In the Cinctae, the plications of the 

 two valves meet at the anterior margin in such manner as to form a straight 

 instead of a crenulated line, as in the Biplicatae. In young specimens the 

 ribs and folds are less prominent and numerous than at maturity. In very old 

 or senile individuals the shell is usually thickened and obese, and the growth 

 lines are much crowded anteriorly. At this stage inherited specific characters 

 are seen to disappear, and at the same time new ones may be introduced. 



Shell Structure. — The test of Brachiopods is composed of laminae of various 

 structure and composition, but differs considerably from that of Molluscs. 

 The shell may be wholly calcareous or alternately calcareous and corneous. 

 When entirely calcareous the laminae are never more 

 than three in number : an inner thick prismatic layer, 

 an intermediate laminar layer, and an outer epidermal 

 film. The inner layer is made up of flattened prisms of 

 calcite arranged parallel to one another with great regu- 

 larity, and forming an acute angle with the surface of the 

 shell (Fig. 526). In the Thecidiidae these fibrous prisms 

 are so intimately united with one another that the shell 

 substance appears almost homogeneous. Very often the 

 fibrous layer is perforated by a series of minute canals 

 which pass from one surface of the valve to the other 

 in a more or less vertical direction, and are somewhat 

 dilated externally. These canals contain tubuli, or certain 

 prolongations derived from the mantle, but never com- 

 municate with the exterior, owing to the fact that the 

 laminar layer of the shell is always covered with a 

 chitinous epidermis (periostracum). With the aid of a magnifier the openings 

 of these canals are visible in fossil forms, and they may be also seen in recent 

 specimens after the epidermis has been removed by an application of caustic 

 potash (Fig. 527). According to the presence or absence of tubuli, Brachiopods 

 are distinguished as punctate or impunctate. 



The Craniidae have thick shells composed of concentric layers of carbonate 



Fio. 526. 



Prismatic fibrous struc- 

 ture of shell ofR/)2/ncfto;te??« 

 (Hemithyris) psittacea, lOOy'j 

 (after Carpenter). 



