Article: Shell development in Spirifer trigonalis from the Carboniferous of Scotland
The neotype of Spirifer trigonalis (Martin) is described and a group of spiriferids from the Carboniferous Limestone Series of Lanarkshire, displaying wide variation in external form, is assigned to this species. The microstructure and distribution of lamellar, fibrous, and inner columnar layers is described. Extracellular secretion of the columnar layer is postulated. Interlayering of fibrous and columnar layers is attributed to pauses in growth, accompanied by retraction of the mantle and a consequent change in distribution of the layers. The distribution and development of internal plates is described. Teeth, dental flanges, delthyrial plate, dental sockets, crura, and cardinal process appear in the neanic shell and continue to grow. The delthyrial plate covers early dental flanges: similarly the cardinal process buries early socket cavities and crural bases. Dental plates appear first in the ephebic shell and contribute to the apical infilling in the adult shell.