12 UNITED STATES NATIONAL MUSEUM BULLETIN 29 6 



The most obvious features of any acrothoracican body, when viewed 

 alive, are the lips of the mantle aperture. The thickened plates are 

 probably homologous and certainly analogous to the occludent margin 

 of the mantle of the thoracicans and can be termed an "operculum." 

 They often are armed with a variety of teeth, spines, hooks, and hairs 

 or bristles. These major structures often are compounded in turn Avith 

 their own serrations, hairs, teeth, and spines. 



In general, a hair or bristle can be defined as a more or less flexible, 

 pointed process ten or more times as long as wide, without additional 

 ornamentation unless it is plumose, when it will have extremely fine 

 hairs along its length. A spine is stouter, with a single major point, 

 but may bear several types of ornamented features, including smaller 

 spines, teeth, or hairs. Teeth usually terminate in more than one point, 

 although the tiny processes scattered around the general mantle 

 surface are called teeth even if some have only one point. Hooks are 

 spines with the distal end curved at least 90 degrees, although the 

 actual curvature often is difficult to estimate. 



Some spines (e.g., Lithoglyptes spinatiLS, L. bicornis?) can be articu- 

 lated, and fold down into the mantle aperture or outward to project 

 beyond the surface of the shelly host material. 



It has already been noted that no acrothoracican hangs in its 

 burrow by the use of these hooks or spines. They are apparently for 

 defense against invasion into the mantle cavity or into the burrow. 

 No foreign animal species has ever been reported with a living female 

 in either the mantle cavity or the burrow. Those species without 

 major hooks and spines on their mantle have opercula which fit 

 tightly into the burrow aperture, closing the connection of the burrow 

 to the outside. 



An orificial knob is seen in some species as a raised hillock, often 

 bearing additional spines, just dorsal to the aperture. The function 

 of this armed knob apparently is to close the area of the burrow 

 aperture not directly occluded by the opercular lips. This arrangement 

 is necessary where the attachment area begins near the surface of the 

 shell and extends posteriorly along the horny knob or disk, for the 

 operculum, to have any flexibility in opening, must be removed from 

 this fixed cemented area by at least a little distance. The orificial 

 knob fills the unprotected gap between cemented attachment area and 

 flexible operculum. The calcareous residue from the abrading process 

 often forms an aggregated mass at the attachment end of the 

 aperture that seems to replace the need for an armed orificial knob, 

 and serves as a mechanical barrier against ingress in this critical non- 

 flexible area. In the Cryptophialidae, where the attachment area has 

 been moved down and away from the burrow aperture as the animal 

 matures and where the very small burrow aperture is filled with the 



