"In the course of work on the taxonomy and evolution of muricid gastropods, we have noted a peculiar feature of the shell surface. It differs from the underlying shell in being flat white in color, much softer, and, in many cases, with intricate sculpture which may not correspond to that of the underlying shell. For this surface layer we have coined the term intritacalx, a name which reflects some of the unique features of this structure: intrita - crumbly; calx - chalk.

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We have studied the intritacalx in 4 families of gastropods (Muricidae, Bursidae, Cancellariidae, Turritellidae) and two families of bivalves (Mactridae, Pholadidae). In most instances it is deposited in the form of simple axial growth striae, differing from the underlying shell only in hardness and color. Where the intritacalx is deposited in axial lamellae, it is not only softer than the underlying shell but also may not correspond to the shell sculpture underlying it. The most unusual form taken by the intritacalx is found in the genera Aspella, Typhisopsis, Tripterotyphis, and related groups, and in the Bursidae. In these groups it is deposited in intricate patterns which are either much exaggerated reflections of the sculpture of the shell beneath it or are completely unrelated to it. The patterns are commonly reticulate, as in Dermomurex and Bursa.

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In conclusion, we believe that the intritacalx is of potentially great taxonomic importance in the groups in which it occurs. This is particularly true in the genera Aspella, Dermomurex, Typhisopsis and Tripterotyphis. In each of these groups the sculpture of the intritacalx is characteristic and constant. It is also of value at the species level, especially in Aspella. In this genus, species from widely separated geographical regions have often been confused and considered conspecific on the basis of worn shells. The distinctive details of the intritacalx of Aspella species are helpful in separating them. On the basis of the foregoing we suggest that: 1: As the intritacalx is mineralogically similar to or identical with the underlying shell, its softness is probably due to a sparsity of organic binding matter. 2: Since the shell is deposited by the mantle, it is not unreasonable to assume that the intritacalx is also laid down by the mantle. 3: We believe that the intritacalx is deposited synchronously with the underlying shell, an assumption strengthened by its deposition, in many cases, immediately under a periostracum."