Posted: May 14 2010
Retilaskeya bicolor (C. B. Adams, 1845) and
Retilaskeya emersonii (C. B. Adams, 1839)
The most ready distinguishing character between these two is the protoconch. The protoconch of R. emersonii consists of a smooth half whorl followed by three axially-ribbed whorls as compared to the protoconch of R. bicolor, which consists of a smooth half whorl followed by two axially-ribbed whorls.
R. bicolor R. emersonii
However, very often protoconchs are extremely worn, incomplete or missing entirely. So, a close examination of the spiral structure can also help distinguish. Redfern notes that the subsutural cord in R. bicolor is brown. Both species display two beaded spiral cords in the early teleoconch whorls and then three in later whorls. I’ve noted that in R. bicolor the beading on the upper cord is distinctly smaller than the lower, whereas in R. emersoni the beading on the upper chord is the same size as the lower (or only slightly smaller). It is also apparent that R. bicolor has more beads per whorls. These two photos are of the same growth point for each species. The top most whorl is the last one with two cords and the next is where the third, middle cord first appears.
Posted on Conch-L, 10/9/14:
"Dear connoisseurs of conchological minutiae,
I'm wondering whether Retilaskeya emersonii occurs in the Caribbean, as R. bicolor surely does.
Malacolog lists distribution of emersonii from Massachusetts, New York, North Carolina, while Harry Lee (2009, p. 88) says it's found in Southeast and West Florida. Marlo Krisberg's excellent presentation shows the differences between emersonii and bicolor at Let's Talk Seashells at the bottom of the page."
"Rolán & Espinosa* recorded two live-collected specimens of R.emersoni (sic) from Cienfuegos, Cuba.
*Rolán, E. & J. Espinosa. 1992. La familia Cerithiopsidae H. y A. Adams, 1853 (Mollusca, Gastropoda), en la Isla de Cuba. 1. El genero Retilaskeya Marshall, 1978. Publ. Ocas. Soc. Port. Malac. 16:39-44."
"Dear other connoisseur,
Rolán and Espinosa (1992) placed the two topical taxa, presently the only members of the genus in the western Atlantic, on firm ground. They found both in Cuba and produced SEM's to prove it.
Although I cited their work, (Lee*, 2009: 88; 179), Marlo may have discovered the protoconch differences independently or simply forgot to cite the previous work of the Spaniard and Cuban.
*Lee, H.G., 2009. Marine Shells of Northeast Florida. Jacksonville Shell Club, Inc. 204 pp. + 19 color plates. 28 May.
"Thanks, Colin and Harry. It appears the ranges of the two species are overlapping, with R. emersonii reaching north to at least Massachusetts."