Having had some experience in shell-collecting during his schooling in Swansea, the Welsh lawyer John Gwyn Jeffreys devoted his spare time to the study of conchology. Becoming an expert in British and Mediterranean Mollusca, Jeffreys is most famous for his five-volume study British Conchology, published between 1862 and 1869 by the preeminent natural history publisher of the nineteenth century, John van Voorst.
Believing that modern molluscs were the descendants of those that lived in British seas during the late Tertiary and early Pleistocene periods (23–2 Mya), Jeffreys made several visits to Shetland in the 1840s but the results were limited. However, between 1861 and 1868 he undertook further deep-sea dredging expeditions in the waters off Shetland with the naturalists Edward Waller, A. M. Norman and others. These later expeditions discovered that climate change had affected the region in the deep past, as the remains of species from colder waters were found among those from warmer seas.
Jeffreys built up a significant collection of shells over his career, including many type specimens. These were later sold to the Smithsonian Institute and shipped to the United States of America.