. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 67. Baculites sp. A-C. Geological Survey collections, Denver, locality D2140, from the Pierre Shale, Campanian, m above the top of the Groat Sandstone at Mesozoic locality D2140, km north of Belle Fourche in SWU SBlA sec. 22, T. UN., R. 2 E., Butte County, South Dakota. The specimen has been coated with ammonium chloride and shows the narrow groove on the internal mould that corresponds to the ridge on the inside of the adapical end of the body chamber that defines the adaper
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 67. Baculites sp. A-C. Geological Survey collections, Denver, locality D2140, from the Pierre Shale, Campanian, m above the top of the Groat Sandstone at Mesozoic locality D2140, km north of Belle Fourche in SWU SBlA sec. 22, T. UN., R. 2 E., Butte County, South Dakota. The specimen has been coated with ammonium chloride and shows the narrow groove on the internal mould that corresponds to the ridge on the inside of the adapical end of the body chamber that defines the adapertural edge of the bilobed dorsal muscle scar. The groove adapertural to the final septum corresponds to the position of the postseptal prismatic zone of Henderson (1984). x 1. of the subepithelial sheath' (1984: 480). Henderson illustrated and described pre- and post-septal prismatic zones, the latter in his view the site of muscle attachment (1984, text-fig. 9). Through the courtesy of Dr W. A. Cobban ( Geological Survey, Denver), we have been able to examine a series of rare and exceptionally preserved Baculites from the US Western Interior that show detail of muscle scars and related features with great clarity. Only a tiny percentage of generally adult Baculites show muscle scars; development is variable even in individuals from a single concretion. The post-septal prismatic zone of Henderson forms a distinct groove, adapertural to the septa (Fig. 67). There is a well-developed adaperturally biconvex groove on the mould (Figs 67-71), corresponding to a ridge of shell material on the inside of the body chamber, that follows the general line of the U/I saddle on the dorsum, and runs back towards the trace of the post-septal prismatic zone. This bilobed trace is presumed to represent the fused dorsal muscle scars described in other ammonites. Much rarer, and. Please note that these images are extracted from scanned page images that may have been digitally enhanced for read
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