19. Segment and Geographic Information

We have two reportable segments organized according to our products and services: Medical Devices and Preservation Services. The Medical Devices segment includes external revenues from product sales of BioGlue, BioFoam, JOTEC products, On-X products, CardioGenesis cardiac laser therapy, PerClot, PhotoFix, HeRO Graft through the second quarter of 2016, and ProCol through the date of the sale of the ProCol product line in the first quarter of 2016. The Preservation Services segment includes external services revenues from the preservation of cardiac and vascular tissues. There are no intersegment revenues.

The primary measure of segment performance, as viewed by our management, is segment gross margin, or net external revenues less cost of products and preservation services. We do not segregate assets by segment; therefore, asset information is excluded from the segment disclosures below.

The following table summarizes revenues, cost of products and preservation services, and gross margins for our operating segments (in thousands):


	2018		2017		2016
Revenues:
Medical devices	$	187,394	$	119,631	$	113,992
Preservation services		75,447		70,071		66,388
Total revenues		262,841		189,702		180,380

Cost of products and preservation services:
Medical devices		53,772		29,798		28,033
Preservation services		36,085		31,262		33,448
Total cost of products and preservation services		89,857		61,060		61,481

Gross margin:
Medical devices		133,622		89,833		85,959
Preservation services		39,362		38,809		32,940
Total gross margin	$	172,984	$	128,642	$	118,899

Net revenues by product for the years ended December 31, 2018, 2017, and 2016 were as follows (in thousands):


	2018		2017		2016
Products:
BioGlue and BioFoam	$	66,660	$	65,939	$	63,461
JOTEC		63,341		4,136		--
On-X		44,832		37,041		34,232
CardioGenesis cardiac laser therapy		6,217		6,866		7,864
PerClot		3,767		3,533		4,021
PhotoFix		2,577		2,116		1,871
HeRO Graft		--		--		2,325
ProCol		--		--		218
Total products		187,394		119,631		113,992

Preservation services:
Cardiac tissue		35,683		32,510		29,697
Vascular tissue		39,764		37,561		36,691
Total preservation services		75,447		70,071		66,388

Total revenues	$	262,841	$	189,702	$	180,380

Net revenues by geographic location attributed to countries based on the location of the customer for the years ended December 31, 2018, 2017, and 2016 were as follows (in thousands):


	2018		2017		2016
U.S.	$	144,651	$	135,102	$	131,727
International		118,190		54,600		48,653
Total revenues	$	262,841	$	189,702	$	180,380

For the year ended December 31, 2018, revenues attributed to customers in Germany accounted for 10% of total revenues as a result of the acquisition of JOTEC in December 2017. During December 31, 2017 and 2016 revenues attributed to customers in any individual country outside the U.S. were less than 10% of total revenues.

At December 31, 2018 and 2017 57% and 60%, respectively, of our long‑lived assets were held in the U.S., where the corporate headquarters and a portion of our manufacturing facilities are located. Our long-lived international assets were $13.1 million as of December 31, 2018, of which 98% were located in Hechingen, Germany. Our long-lived international assets were $13.3 million as of December 31, 2017, of which 99% were located in Hechingen, Germany. At December 31, 2018 and 2017 $188.8 million and $188.3 million, respectively, of our goodwill was allocated entirely to our Medical Devices segment.