Helcionopsis subcarinata
Encyclopedia
Helcionopsis subcarinata is an extinct species of paleozoic monoplacophora
n in the family Tryblidiidae
.
is small and subovate in outline. The anterior and posterior margins are subequal and sharply rounded in the middle. In the cast the apex
is depressed, small and not quite marginal. An obtuse carination extends across the length of the shell. The surface of cast shows remains of very fine radiating lines, that are scarcely visible without a magnifier, and a few obscure lines of growth.
The length of the shell is 10 mm, width 8 mm and the height of the shell is 3.5 mm. The height of the apex is 1.5 mm.
Ulrich and Scofield collected only two specimens of this species. Both are casts and show impressions of the rostral muscles. One exhibits besides a series of muscular scars similar to those of Tryblidium
. They stated: "The lines are converging from them towards the apex are a peculiar feature, though probably of the nature of progressive tracts."
Monoplacophora
Monoplacophora, meaning "bearing one plate", is a polyphyletic class of mollusks with a cap-like shell, living on the bottom of deep sea. Extant representatives were unknown until 1952; previously they were known only from the fossil record.- Definition :...
n in the family Tryblidiidae
Tryblidiidae
Pilina is an extinct family of paleozoic monoplacophorans in the superfamily Tryblidioidea.- Genera :Genera in the family Tryblidiidae include:* Tryblidium Lindström, 1880...
.
Shell description
The shellMollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
is small and subovate in outline. The anterior and posterior margins are subequal and sharply rounded in the middle. In the cast the apex
Apex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
is depressed, small and not quite marginal. An obtuse carination extends across the length of the shell. The surface of cast shows remains of very fine radiating lines, that are scarcely visible without a magnifier, and a few obscure lines of growth.
The length of the shell is 10 mm, width 8 mm and the height of the shell is 3.5 mm. The height of the apex is 1.5 mm.
Ulrich and Scofield collected only two specimens of this species. Both are casts and show impressions of the rostral muscles. One exhibits besides a series of muscular scars similar to those of Tryblidium
Tryblidium
Tryblidium is a paleozoic genus of Ordovician and Silurian monoplacophorans.The generic name comes from the Greek , which means patella.- Species :Species in th genus Tryblidium include:* Tryblidium acutum* Tryblidium arctica...
. They stated: "The lines are converging from them towards the apex are a peculiar feature, though probably of the nature of progressive tracts."