298 MOLLUSCOIDEA SUB-KINGDOM v 



of the shell cavity with sediment. Not infrequently the shell and the 

 brachidia are secondarily replaced by quartz ; and if the interior filling matter 

 be dissolved away by dilute hydrochloric acid, exquisite preparations may be 

 obtained, often revealing the minutest details. Sometimes hollow shells are 

 found, in which the brachidia are well preserved ; but these structures 

 generally are more or less encrusted. It is often necessary to remove the 

 ventral valve, when the infiltrated material can be cut away by the use of 

 proper tools. Success in manipulations of this kind requires not only con- 

 siderable dexterity, but the conditions of preservation must have been very 

 favourable. The brachidia must be perfectly preserved, and the surround- 

 ing matrix must admit of being removed without injury to the specimen. 

 When other expedients fail, recourse can still be had to polishing, the shell 

 being gradually ground down by abrasion with emery powder on a glass 

 plate. The beaks are first ground away, until the first traces of the crura 

 appear ; the surface is then cleaned and kept moistened while a careful 

 drawing is made. Grinding is resumed for a short interval, when the 

 surface is again cleansed and drawn. This process is repeated until the 

 sections include the entire brachial support. From the series of transverse 

 sections thus obtained, the brachidium can be ideally reconstructed. 



The spondylium is an internal ventral plate traversing the posterior 

 portion of the valve (Fig. 538, C). On the superior surface of the plate are 

 inserted the adductors, diductors, and the ventral pedicle muscles. Beneath 

 the spondylium, which may be supported by a median septum, are situated 

 the reproductive organs. The plate is homologous with the solid or ex- 

 cavated platform of the Atremata (Trimerellidae and Lingulasmatidae). 



The Animal : Mantle. Lining the entire inner surface of the shell is a 

 thin, transparent membrane, which appears in the embryonic condition as 

 two distinct lobes of the thoracic segment in the 

 Cephalula stage. This is the mantle or pallium, 

 which is primarily concerned in the secretion of the 

 shell. In Crania it consists of three layers : a middle 

 cartilaginous, an inner ciliated one, and an outer 

 layer of cells. The layer lying against the surface 

 of the shell is often studded with minute caeca or 

 blind tubes, which enter the perforations of the test. 

 __ The mantle (or certain of its layers) is folded upon 



Flo . 495> itself at various points, enclosing cavities or pallial 



HumUetonensis, sinuses, which contain the circulating fluids, and 

 ! frequently portions of the genital organs. Distinct 

 Davidson) palHal sinuscs (aftr i m P ressi ns f these sinuses are often observable in 

 the valves of both recent and fossil specimens (Fig. 



495). In all the greater sinuses of the mantle, in the perivisceral cavity, 

 and in the cavernous brachia and cirri, occur calcareous spicules of various 

 shapes. These are especially abundant in the Thecidiidae, and form an 

 irregular mass or network. They appear to be absent in Magellania, 

 Terebratella, and Lingula. The outer margins of the mantle are thickened 

 and set with numerous, chitinous, simple, or barbed setae, sometimes of great 

 length. 



The shell cavity is divided by a vertical membranous wall, which is an 

 extension of the mantle, into two regions : a posterior, or visceral cavity, 



