MORPHOLOGY OF THE ORTHOID SHELL 29 



3. Spondyloid. — This term is suggested for a structure simulating a spondylium in form 

 but produced by the deposition of adventitious testaceous substance on and about the clental lamellx, 

 swelling them laterally until union is effected. This type of structure is well exhibited in Poram- 

 bowtes. In late mature or gcrontic individuals of this genus, adventitious shell has been deposited 

 on the dental lamellx and on the floor of the valve in front of them, and may be produced forward 

 sufiicicntlv to simulate a median ridge or septum. In old shells this structure so closely resembles 

 the spondylium in pentamerids that Noetling and subsequent workers have been misled into the con- 

 ception that the thickening of the dental plates in Poratnbonites is a spondylium. 



4. Free spondylium. — Here the dental plates converge and unite to form a spoonlike muscle 

 platform that hangs suspended in the ventral valve, since it is not supported by a median septum as 

 in the typical forms of spondylia. This type is exceedingly rare, being known in Protorthts of the 

 Middle Cambrian, and the younger genera Holorhynchus and Cymbidium. 



5. Spondylium or spondylium simplex. — The term spondylium has customarily been 

 applied to the spoon-shaped muscle platform formed by the convergence and union of^^the dental 

 lamellx with a median septum, regardless of the structure of the septum. Kozlowski, however, 

 has shown that the spondylia are differently constructed and are therefore polyphyletic m origin, and 

 has consequently defined several kinds. He shows that the spondylium in Skenidmrn and Cluatnr- 

 bonhes is different in origin from that in the Pentameracea. In Clitambonites the spondylium is 

 formed by the union of the dental plates with a simple median septum (euseptum), the whole 

 forming one piece. To this type of spondylium he gave the name spondylium simplex. As this is 

 the simplest type of supported spondylium, we may call it spondylium and drop the limiting word 



simplex. , , ^,. , • • j j • i 



The spondylium is one of the characteristics of the subfamily Clitambonitidae and is seen also 



in the Syntrophiidas and Skenidiidx. 



6. Duplex spondylium. — The above type of spondylium is in decided contrast to that in the 

 Pentameracea (Conchidium), in which the supporting septum is actually double and composed of two 

 united vertical plates. To the latter type Kozlowski has applied the name spondylium duplex, and it 

 appears not to occur in any genus of the Orthoidea," but is characteristic of the pentamerids, where it 

 is associated with a definite kind of crural apparatus. ■ r . j 



In its evolution, Kozlowski believes that the spondylium simplex has arisen from the pseudo- 

 spondylium. The genital organs are thought to have resorbed the testaceous deposit about the base 

 of the dental plates and under the muscle attachments. The resorption has gone so far as to leave 

 a rather narrow septum underneath the seat of muscle attachments. This is, in its essentials, precisely 

 the same idea held by Schuchert in 1897.^^ The spondylium duplex, however is considered by 

 Kozlowski to have originated in a different manner, i. e., through lateral crowding by the sexual 

 glands, which has pushed the dental lamellx gradually together. 



SEPTA 



Vertical septa are usually not conspicuous features of the ventral interior, but in sonie genera 

 rather prominent ridges or septa are developed in connection with the musculature, deltidium, and 

 pedicle attachment. Such ridges usually have value in species making but only rarely have they 

 generic significance. . . , ,, 



In shells wherein a pedicle callist or a deltidium is present, it is not uncommon tor a small 

 septum to extend forward from the callist or deltidium for a short distance, dividing the back ends 

 of the diductor scars. This ridge is commonly formed in old shells and is the result of deposition of 

 adventitious shell between the muscles. Such septa are, however, of little taxonomic significance. 



" 1929, pp. 122-126. ..,.,, l j . 



"The writers have not seen a thin section of the spondylium of EnteUtella and lU detailed nature can not be deter- 

 mined from Likarev's figure. 

 "Op. cit., pp. 99-100. 



