Thyone and Havelockia are worldwide-distributed genera represented in the Western Atlantic by eight and three species, respectively. In this paper, we describe two new shallow-water species from the Brazilian coast: Thyone waltinhoi sp. nov. and Havelockia oraneae sp. nov. These genera are very similar morphologically but the presence of subdivisions in their calcareous rings separates them into different families, Thyone in Phyllophoridae and Havelockia in Sclerodactylidae. To observe the presence of subdivisions, we submerged the calcareous rings in an iodine solution prior to morphological analyses. Thyone waltinhoi sp. nov. differs from its Western Atlantic congeners in its dermal ossicles and Havelockia oraneae sp. nov. differs from its South-western Atlantic congeners in the composition of its introvert ossicles. In addition to species descriptions, illustrations and morphological diagnoses of both genera are provided, along with a synoptic table of their Western Atlantic species, and a discussion of the taxonomic significance of the dermal ossicles and the calcareous ring.

Thyone pawsoni and Holothuria (Theelothuria) princeps are reported from shallow water of the South Atlantic Ocean. A morphological description of these new records with colour figures and scanning electron microscopy images of their ossicles are provided. With these two new records for the Brazilian Coast, the genus Thyone is now represented by two species and Holothuria by six.

A new species of a dendrochirotid holothuroid, Thyone herberti, is described from the deep waters of Matigulu, KwaZulu-Natal, South Africa. The new species is distinct from other species of the genus in the form of its calcareous ring, in which the radial plates are prolonged posteriorly before bifurcating, and in the presence of non-rosette-like perforated rods and plates in the tentacles and the introvert. The new species increases to eight the number of southern African species currently classified in the genus Thyone.