Davidson, in a paper on the musculature of Heptanchus maculatus (1918), mentions a small shield-shaped organ to be found in the parietal fossa, and in connection with it a pair of small muscles having their origin on the cranium and dorsal longitudinal muscles. He believes that these muscles constrict this sac-like organ.
Proceedings of the Iowa Academy of Science
©1927 Iowa Academy of Science, Inc.
Smiley, R. E.; Ingram, W. R.; and Blagg, E.
"The Organs of the Parietal Fossa in Elasmobranchs,"
Proceedings of the Iowa Academy of Science, 34(1), 315-317.
Available at: https://scholarworks.uni.edu/pias/vol34/iss1/109