This widely-spread, and, on the whole, well-marked species, has been in so many cases, as it appears to me, confounded with Alaria bispinosa, that it would be a task of no small difficulty to trace out the mistakes that have arisen in consequence. The imperfect figures of Phillips have, no doubt, been the principal cause of thus confounding the two species. Deslongchamps queried the identification, but yet was inclined to adopt it. Morris and Lycett seem to have had no doubt on the subject. “Having had the advantage of examining a large number of specimens, comprising every variety both in form and stage of growth, we feel no hesitation in uniting the two species here indicated.” It is extremely probable that the Great Oolite specimens of Alaria trifida present varietal differences from the Oxfordian types, and they may also in some cases have assumed forms having only one lateral digitation; but higher up in the Jurassic rocks they seem to be very fairly separable for such a genus as Alaria, which is so prone to effect changes in its shell. Such was doubtless the opinion of Morris, who quotes Al. bispinosa from the Lower Calc. Grit and Kel. Rock, whilst Al. trifida is quoted as a distinct species from the Oxford Clay and Kel. Rock of Yorkshire, the Great Oolite form being referred to a variety.