174(5 THE VOYAGE OF H.M.S. CHALLENGER. 



but are in loose contact in the equatorial plane ; here the free edges of both valves catch into 

 one another by means of free ramules (PL 128, figs. 1, 7). This loose connection is 

 similar to what occurs in the Conchopsida (or in the Concharida with dentate edges), but 

 never so regular. The special form of the polyhedral lattice-mantle depends on the 

 number, arrangement, and development of the styles, which proceed over its surface ; it 

 preserves the polyhedral form of the calymma, on the surface of which it is deposed. 



The characteristic styles of the Coelographida (which are never found in the preceding 

 Ccelodendrida) are longer hollow tubes, symmetrically disj)osed on both valves. They 

 are prominent over the surface of the fork-thicket in the Ccelotholida, of the lattice-mantle 

 in the Coeloplegmida. They bear in these latter a peculiar terminal coronet on their distal 

 end, whilst in the former this end is armed with large pencils of spathUlse. The styles 

 may be forked once or twice at their base, but in their greatest part they are verticillate, 

 and not dichotomously branched like the brushes. The lateral branches of the styles 

 are usually very numerous and regularly cruciate in alternating opposite pairs. In the 

 odd nasal style, e.g., the first and third pairs of opposite lateral branches usually lie in 

 the frontal plane, the second and fourth in the sagittal plane, perpendicular to the former, 

 and so on. A similar regular disposition of the lateral branches is found also in 

 other styles, but not in all. There are certain styles in which the lateral branches are 

 not opposite in pairs, but alternate or verticillate, and others in which they represent 

 unequal branches of forks, so that each single segment of the branched style represents 

 the stouter branch of a fork, and the appertaining lateral branch the thinner branch of 

 the fork. Further accurate examinations are required to recognise the different laws of 

 the ramification of the styles in the different forms of Coelographida. The lateral 

 branches of the styles are usually again dichotomously branched inside the lattice-mantle, 

 and their distal ends pass over into its network. But the verticillate or cruciate branches, 

 which arise from the free part of the styles outside the lattice-mantle, are always armed 

 with the same elegant pencils of spathillse which cover the surface of the fork-thicket in 

 the Ccelotholida, the surface of the lattice-mantle in the Coeloplegmida. These pencils 

 also are often regularly opposite in pairs, and the pairs alternate in two planes perpen- 

 dicular one to another (PL 128, figs. 1, 4). 



The terminal coronets are peculiar ornaments which protect the distal ends of the 

 styles in the Coeloplegmida, whilst in the Ccelotholida these are armed with the usual 

 pencils of spathillse (PL 122, fig. 8). Each coronet is usually produced by the double, 

 triple, or quadi'uple furcation of the free distal end of the style ; therefore composed of 

 four, eight, or sixteen terminal branches, which, on account of their peculiar form and 

 function, we may call " fingers." More rarely the ramification of the coronets is more or 

 less irregular, and sometimes the number of the fingers exceeds twenty or even thirty. 



In the majority of species eight fingers are regularly disposed (PL 127, figs. 1-3; 

 PL 128, figs. 1-8). Often too sixteen occur, rarely four only. Sometimes the fingers 



