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Summary.

The following are a few of the most important points dealt with in the paper:—

  • 1
    The Membranous Labyrinth is described and figured for the first time, and it has been found to be of the usual selachian type, resembling rather closel that of Hexanchus griseus.
  • 2
    In the formation of the centra the following points are especially worthy of note:—
  • (a) 
    The preaence, at the anterior end of the vertebral column, of well-developed, calcified cyclospondylic centra.
  • (b) 
    Slight constrictions of the notochord in the trunk region, best, seen in horiqontal longitudinal section, not calcified. The lowest stage in the development of centra.
  • (c) 
    In the main caudal region, calcified cyclospondylic centra of two sizes are present, corresponding with the doubling in the number of arcualia, which here, more than elsewhere in the vertebral column, grow round the notochord and greatly strengthen the centra.
  • 3
    The terminal caudal region is heterospondylic, and not monospondylic as in many other Selachians.
  • 4
    The musculature and skeleton of the mixipterygia are dealt with fully for the first time. In regard to the musculature I have to record the presence of both musc. fleœor internus and musc. flexor externus of Huber. In the skeleton I have found the modified radial β, and only one small intercalary cartilage b.1, not three as recorded by Günther.