Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/499



but from this it differs in having the median teeth circular, instead of transversely oval.

Another specimen belonging to Mr. Mansel is one of great interest. There has long been a controversy touching the validity of the genus Sphoerodus. The similarity of the teeth to those of some of the larger species of Lepidotus induced Agassiz to establish the genus with some hesitation. Owen, however, detected some differences in the microscopic structure of the tooth, which led him to consider the two genera distinct. Many continental palaeontologists, on the other hand, repudiated the genus Sphoerodus, as founded in error. The main obstacle to a satisfactory solution of the difficulty arose from the circumstance that the Bufonites were generally found detached from the dentary bones. In Mr. Mansel's specimen (fig. 5), however, we have true Sphoerodus teeth, arranged

Fig. 4. — Vomer of Gyrodus coccoderma.

Fig. 5. — Vomer of Sphaerodus gigas.

in natural order ; and not only so, but the specimen is fortunately a vomerine bone, the form of which is entirely different from the palatal organization of Lepidotus, and essentially characteristic of the Pycnodonti. The specimen is 2-1/2 inches in length, and contains the median series of teeth, the two intermediate rows, and two teeth of the marginal row of the left side. The median row is composed of six teeth of circular form ; the intermediate rows con-