126 DESCRIPTIONS OF PREPARATIONS. 



In those Lamellibranchiata which possess siphons, the pallial line is incurved 

 beneath the impression of the posterior adductor muscles, forming a bay or sinus. 

 Such shells are said to be sinupalliate. 



The epicuticula varies in thickness in different Lamellibranchiata^ and it is 

 sometimes complicated in structure as in Mytilus edulis. It is rather thin in Ano- 

 donta. The prismatic layer consists of more than one layer of calcareous prisms, 

 which vary in size and external shape. The different layers of prisms are held 

 together by remains of the organic substance (conchiolin) in which the calcareous 

 prisms are deposited. At their first appearance these bodies are, in Anodonta 

 at least, more or less rounded and irregular in shape, and at some distance apart. 

 They increase in size, and at the same time new prisms appear between those 

 first formed. In some instances, e. g. Cyprina islandica, the prismatic layer consists 

 chiefly of a mass of granular calcareous matter : in other instances it is very 

 distinctly lamellate, like the nacre, e. g. Astarte borealis. It may closely resemble the 

 Gastropod shell, and consist of vertical laminae, e. g. Cardium. The nacre consists 

 of alternating lamellae of conchiolin, and of conchiolin containing calcareous deposits, 

 which appear exactly in the same manner as the prisms of the prismatic layer. 

 In the region of the various muscular impressions it is ' transparent,' and here the 

 muscular fibres are directly continuous with the shell-substance, and not separated 

 from it by an epidermis. According to F. Miiller, the inner surface of the nacre 

 is covered by a soft layer. 



The Cydas cornea of the freshwaters differs from other Lamellibranchiata in the 

 structure of its shell. There is no prismatic layer, and the organic portion of the 

 nacreous layer is reticulate. Short wide unbranched canals extend into the sub- 

 stance of the shell and are lined apparently by extensions of the mantle. In this 

 point they differ from the branching canals so often found in the substance of the 

 shell in other Lamellibranchiata^ which, according to Kolliker, are due to algal 

 parasites. Ehrenbaum, however, who mentions them in various genera, says 

 nothing as to their contents. 



The structure of the calcareous parts of the Lamellibranch shell does not 

 appear to be by any means so uniform as is often supposed. 



The ligament in Anodonta consists of an outer and inner part. The outer 

 is laminated and passes gradually into the epicuticula. The inner is striated 

 radially, and consists of radial fibre composed of two different alternating sub- 

 stances refracting light differently. Hence in cross sections this inner part appears 

 to be laminated like the outer : but the mode in which it splits proves its real 

 radial structure. It must probably be regarded as continuous with the nacreous 

 layer. Where it borders the nacreous layer internally there is a distinct nacreous 

 ridge, the homologue in Anodonta of the region which is produced into teeth in 

 Unto, &c., and, like the teeth, giving attachment to muscles ascending from the foot 

 (F. Miiller). The border of the ligament itself is connected with numerous 

 isolated bundles of muscles ; and just as the * transparent ' portion of the nacre in 

 the muscular impressions is continuous with muscle fibres, so here the fibres of the 

 inner part of the ligament are continuous with muscle fibres. 



The relations of the ligament to the shell- valves show that, strictly speaking, the 

 valves ought to be regarded as parts of a continuous structure. The dorsal region 

 of this structure does not undergo calcification, or only to a very slight extent, inas- 



