GENERA OF THE SUBORDER ORTHOIDEA 



151 



prominent, subparalkl, produced forward as prominent 

 ridges on each side of the diductor scars; a median 

 ridge extends from the front of the muscle area nearly 

 to the margin of the shell and is commonly extended 

 between the diductor impressions nearly to the apex. 

 Muscle field longer than wide; adductor scars borne 

 on the median ridge; diductor scars elongate, narrow; 

 muscle area thickened in front; irregular, wavy 

 ovarian markings occupying the lateral spaces. The 

 palintrope overhangs the delthyrial cavity. 



Dorsal interior. — Cardinalia strong; notothyrial 

 cavity deep; brachiophores forming a thickening below 

 the notothyrial margin and terminating in short, blunt 

 points supported by stout plates that converge to meet 

 a median septum and thus make a cruralium; sockets 

 shallow, excavated in the margin of the hinge and the 

 outer face of the crural base, defined by an obscure 

 fulcral plate; cardinal process unique, consisting of an 

 elongate thickened myophore and a thin septum-like 

 shaft that is continuous with the median septum. 

 Median septum high, extended forward about half 

 the length of the valve. Muscle area elongate-oval, 

 divided by the median septum, commonly with an ele- 

 vated periphery. Elevated, wavy visceral markings 

 occupy the spaces outside of and posterior to the 

 muscle area. 



Geologic range. — Middle Silurian or Gotlandian 

 (Niagaran). 



American Species 

 Orthit functostriata Hall 1852 



European Species 

 Orthis functata Verneuil 1 848 

 ? O. turgida McCoy 1 85 I 



Distinguishing characters. — Ltnoforella is 

 unique among punctate orthoid genera in the conver- 

 gence of its ornamentation toward the kind seen in 

 Porambonttes. There are, however, internal and ex- 

 ternal differences that when combined with the geolog- 

 ical occurrences separate the two genera. Externally 

 Linoporella has the outline and profile of Pionodema, 

 differing thus from Porambonttes, and internally the 

 ventral musculature, dorsal cruralium, and cardinal 

 process further serve to differentiate the two. 



Discussion.— Hall and Clarke (ISgZ)'-" were 

 aware of this group of orthids, saying of O. functata 

 and O. punctostriata that they could not be placed in 

 their divisions of the old genus Orthis; they did not, 

 however, give them a generic name. 



The internal features of Linoporella separate the 

 genus from all other Dalmanellacea. In the ventral 

 valve of young shells the dental plates are sharply 

 defined by deep umbonal cavities and extend directly 



'^'Pal. N. Y., vol. 8, pt. 1, p. 217. 



to the floor of the valve. In most specimens these 

 plates are prolonged along the floor of the valve as low 

 ridges. The diductor scars are semielliptical or semi- 

 oval in plan and are separated by a depressed adductor 

 ridge. The latter is low in the posterior of the shell 

 but is elevated to a sharp crest at a point just in front 

 of the anterior ends of the diductor scars, and then 

 descends to disappearance near the front margin. 

 Along the top of this ridge is a shallow, longitudinal 

 groove which usually does not extend anterior to the 

 crest. In older shells the front margins of the diduc- 

 tor scars are elevated on a low callus deposit which is 

 elevated forward and inward along the median ridge 

 and rises to disappearance at the crest. Posteriorly and 

 laterally the callus decreases in thickness to become 

 only a film on the sides of the dental plates and floor of 

 the valve. This callus is significant, however, because 

 it obscures the front ends of the dental plates and 

 produces a pseudospondyliurp. Careful observation of 

 old specimens will commonly show low extensions of 

 the dental plates into or beyond the callus and its 

 upturned anterior border. The delthyrial region of 

 Linoporella is another significant example of obsoles- 

 cence of primary structures (bases of dental plates in 

 this instance) by secondary deposit of shell. In this 

 tendency it also parallels Porambonttes, although the 

 degree to which the deposition is carried does not result 

 in structures that are precisely alike. 



The dorsal interior is also unique in Linoporella, 

 being a convergence to that seen in Skenidio'tdes. The 

 brachiophores are elongate, toothlike, bluntly pointed 

 processes extending into the interior of the valve. 

 Their forward growth produces a thickening under 

 the palintrope which lies on them and extends over 

 the notothyrial cavity for a short distance. These 

 processes merge into discrete convergent plates which 

 unite near the floor of the valve with a sharp median 

 septum. In old shells the cavities between the brachio- 

 phore supports and the floor of the valve are filled up 

 with adventitious shell, causing the development of a 

 structure simulating a sessile cruralium. In reality, 

 however, in young shells the brachiophore supports in 

 their line of attachment with the septum curve ventrally 

 so that their anterior ends rise to a point on the summit 

 of the septum. The cardinal process has a simple shaft 

 that thickens toward the front and merges with the 

 median ridge to form a continuous septum, the thicken- 

 ing at the front of the cruralium being the only mark 

 of union of the two septa. The myophore in old shells 

 is a triangular thickening of the shaft at the posterior, 

 the compressed sides of the triangle bearing the crenu- 

 lations or marks of muscle attachment. The median 

 ridge usually extends to about the middle of the valve 

 as is usually the case in the orthids. The muscle field 

 is rather narrow, elongate oval, with the posterior 

 adductor scars the larger. Each anterior mark is 

 bipartite as in many other orthid genera. Elevated, 



