. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 285 inner sheet nacreous layer about 500 [x thick (Plate 15, figs i & 2). The outer shell surface is ornamented with small granules radiating from the umbo (Plate 15, fig. 5). The outer more irregular part of the saddle shaped area consists of homogeneous structure and is lain down on the inside of the nacreous layer. On the inner surface of the homogeneous layer, is the attachment of the pallial muscles which secrete beneath them, myostracal prisms forming a conspicuous 'W shaped scar. The tube
. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 285 inner sheet nacreous layer about 500 [x thick (Plate 15, figs i & 2). The outer shell surface is ornamented with small granules radiating from the umbo (Plate 15, fig. 5). The outer more irregular part of the saddle shaped area consists of homogeneous structure and is lain down on the inside of the nacreous layer. On the inner surface of the homogeneous layer, is the attachment of the pallial muscles which secrete beneath them, myostracal prisms forming a conspicuous 'W shaped scar. The tube and pot form the most conspicuous part of the animal. Optically the shell structure of these features appears homogeneous with conspicuous lamellate banding. Electronmicroscopy shows that both the tube and pot are made up of platy crystals 0-5-2 [J. in diameter, 0-3-0-5 (x in width irregular in outline but aligned with the long axis parallel with the outside of the tube. (Plate 15, figs 2 & 4). The mode of secretion of the pot and tube pose problems; both of these two structures lie external to the periostracum which is not in intimate contact with the tube but encases the long siphons (Purchon, 1956). On many specimens growth increments can be seen at the posterior end of the tube and this must be formed by valves tube. siphonal aperture periostracum inserted here saddle area Fig. 32. Sketch showing the main features of the shell of Penicillus Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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