Cucullaea is a semi-infaunal, suspension-feeding bivalve with an inflated, box-like, shell. Cucullaea raea from the Eocene of Seymour Island, Antarctic Peninsula, has significantly larger and much thicker shells than modern representatives of the genus collected from what we infer to be similar facies (shallow, shifting sands). Eocene Cucullaea grew in temperate waters, while modern species are restricted to the tropics. Thicker shells at cool, high latitudes would not be predicted by the greater solubility of carbonate at lower temperature, and one might also expect tropical shells to be thicker because of the generally greater predation pressure there, necessitating more defensive architecture in prey species. As modern and ancient species share a similar phylogeny, these differences in shell form must be due to some combination of differences in their environments (hence be adaptive), and/or differences in the way their shells were constructed (life history, growth rate). Here, we explore these possibilities and conclude that adaptation to different physical environments cannot fully explain the observed differences in morphology. Rather, we suggest that the unusual life histories of these clams had a significant influence on their shell form. Growth rates and shell construction of high-latitude clams can be expected to differ from more tropical relatives because they live in darkness for part of the year, during which times their supply of suspended food is presumably limited. Eocene Cucullaea exhibit many pronounced, closely-spaced growth bands that might reflect seasonality of growth, but if so, also imply very slow growth rates and extremely long life spans. Stable isotopic analysis of microsamples of shell material collected parallel to growth lines can reveal whether or not the shell grew slowly over a long period of time or very quickly over a short amount of time, as well as whether growth was continuous or episodic. We use isotopic data from modern and ancient Cucullaea to test hypotheses about growth rates in these high-latitude Eocene clams.